UI: improve unit names

This commit tweaks some unit names:

- Remove `name_alt` for square mile "sq m" and cubic mile "cu m" as
  they could easily be confused with meters.
- Correct plural forms for the ton units: rename "ton" to "tonne" for
  the metric ton, and "tonnes" to "tons" for the imperial (short) ton.
  Identifiers are unchanged.
- Swap `name_short` and `name_alt` for metric ton, hour, and second,
  as `name_short` is used for UI display and should use the official
  symbol. Keep the other form as `name_alt` for input.
- Use "t" and "tn" respectively as short names for the metric and
  imperial (short) ton.
- Rename radian's short name "r" to "rad", keep it as alt name.
- Introduce alt names for km/h (kph), arcminutes (amin),
  arcseconds (asec), for convenience.

References:
- https://en.wikipedia.org/wiki/Ton
- https://en.wikipedia.org/wiki/Tonne

Ref: !116762

source/blender/blenkernel/intern/unit.cc

@@ -492,7 +492,7 @@ static bUnitDef buImperialAreaDef[] = {
         /*name*/ "square mile",
         /*name_plural*/ "square miles",
         /*name_short*/ "sq mi",
-        /*name_alt*/ "sq m",
+        /*name_alt*/ nullptr,
         /*name_display*/ "Square Miles",
         /*identifier*/ nullptr,
         /*scalar*/ UN_SC_MI *UN_SC_MI,
@@ -680,7 +680,7 @@ static bUnitDef buImperialVolDef[] = {
         /*name*/ "cubic mile",
         /*name_plural*/ "cubic miles",
         /*name_short*/ "cu mi",
-        /*name_alt*/ "cu m",
+        /*name_alt*/ nullptr,
         /*name_display*/ "Cubic Miles",
         /*identifier*/ nullptr,
         /*scalar*/ UN_SC_MI *UN_SC_MI *UN_SC_MI,
@@ -766,10 +766,10 @@ static bUnitCollection buImperialVolCollection = {
 /* Mass. */
 static bUnitDef buMetricMassDef[] = {
     {
-        /*name*/ "ton",
+        /*name*/ "tonne",
         /*name_plural*/ "tonnes",
-        /*name_short*/ "ton",
-        /*name_alt*/ "t",
+        /*name_short*/ "t",
+        /*name_alt*/ "ton",
         /*name_display*/ "Tonnes",
         /*identifier*/ "TONNES",
         /*scalar*/ UN_SC_MTON,
@@ -855,10 +855,10 @@ static bUnitCollection buMetricMassCollection = {
 static bUnitDef buImperialMassDef[] = {
     {
         /*name*/ "ton",
-        /*name_plural*/ "tonnes",
-        /*name_short*/ "ton",
-        /*name_alt*/ "t",
-        /*name_display*/ "Tonnes",
+        /*name_plural*/ "tons",
+        /*name_short*/ "tn",
+        /*name_alt*/ nullptr,
+        /*name_display*/ "Tons",
         /*identifier*/ "TONNES",
         /*scalar*/ UN_SC_ITON,
         /*bias*/ 0.0,
@@ -939,7 +939,7 @@ static bUnitDef buMetricVelDef[] = {
         /*name*/ "kilometer per hour",
         /*name_plural*/ "kilometers per hour",
         /*name_short*/ "km/h",
-        /*name_alt*/ nullptr,
+        /*name_alt*/ "kph",
         /*name_display*/ "Kilometers per hour",
         /*identifier*/ nullptr,
         /*scalar*/ UN_SC_KM / 3600.0f,
@@ -1050,8 +1050,8 @@ static bUnitDef buNaturalTimeDef[] = {
     {
         /*name*/ "hour",
         /*name_plural*/ "hours",
-        /*name_short*/ "hr",
-        /*name_alt*/ "h",
+        /*name_short*/ "h",
+        /*name_alt*/ "hr",
         /*name_display*/ "Hours",
         /*identifier*/ "HOURS",
         /*scalar*/ 3600.0,
@@ -1073,8 +1073,8 @@ static bUnitDef buNaturalTimeDef[] = {
     {
         /*name*/ "second",
         /*name_plural*/ "seconds",
-        /*name_short*/ "sec",
-        /*name_alt*/ "s",
+        /*name_short*/ "s",
+        /*name_alt*/ "sec",
         /*name_display*/ "Seconds",
         /*identifier*/ "SECONDS",
         /*scalar*/ 1.0,
@@ -1129,7 +1129,7 @@ static bUnitDef buNaturalRotDef[] = {
         /*name*/ "arcminute",
         /*name_plural*/ "arcminutes",
         /*name_short*/ "'",
-        /*name_alt*/ nullptr,
+        /*name_alt*/ "amin",
         /*name_display*/ "Arcminutes",
         /*identifier*/ "ARCMINUTES",
         /*scalar*/ (M_PI / 180.0) / 60.0,
@@ -1140,7 +1140,7 @@ static bUnitDef buNaturalRotDef[] = {
         /*name*/ "arcsecond",
         /*name_plural*/ "arcseconds",
         /*name_short*/ "\"",
-        /*name_alt*/ nullptr,
+        /*name_alt*/ "asec",
         /*name_display*/ "Arcseconds",
         /*identifier*/ "ARCSECONDS",
         /*scalar*/ (M_PI / 180.0) / 3600.0,
@@ -1150,8 +1150,8 @@ static bUnitDef buNaturalRotDef[] = {
     {
         /*name*/ "radian",
         /*name_plural*/ "radians",
-        /*name_short*/ "r",
-        /*name_alt*/ nullptr,
+        /*name_short*/ "rad",
+        /*name_alt*/ "r",
         /*name_display*/ "Radians",
         /*identifier*/ "RADIANS",
         /*scalar*/ 1.0,