UI: Theme: Common colors for animation channels

Continuation of #140360

New common properties for Channel and Channel Selected,
used in animation editors.
Previously Dope Sheet, Graph Editor, NLA, and Movie Clip Editor
had separate properties for them.

Video and images inside the PR.

---

> [!important]

- Channels looked different in Dope Sheet and Graph Editor. In the former,
they were same color as the background, so only labels and icons were visible,
in the latter they had a darker color, so their outline was visible. We need to pick
which design we go for now that they're combined. I chose color from Graph
Editor for now, with alpha from Dope Sheet, since only that editor uses alpha.

---

There is a confusion with names of channel colors. In 4.5 there is:
- - "Channel" (singular, blue color) which refers to objects, actions,
and action slots that hold/combine channels.
- - "Channels" (plural, black color) which refers to individual channels.

This goes against how we name things. In theme, especially, we refer to object
colors as "Object", singular, not Objects, plular. Same goes for all elements except
this one, where individual elements are referred in plural.

To lessen the confusion, and also avoid unnecessary breaking changes in the
future (in case we want to rename blue things, which seems likely), in this PR
I swap those names. Main reason is that black color marks individual channels,
so it should be singular, and blue colors are combination of many channels,
so they should be plural. Otherwise even talking about them is awkward.

Note that renaming isn't a breaking change, because we already "broke" them
by moving them to common path from individual editors.

Pull Request: https://projects.blender.org/blender/blender/pulls/146091

release/datafiles/userdef/userdef_default_theme.c

@@ -298,10 +298,12 @@ const bTheme U_theme_default = {
       .preview_range = RGBA(0xa14d0066),
       .time_marker = RGBA(0xffffff4d),
       .time_marker_selected = RGBA(0xffffffb3),
-      .channel = RGBA(0x194e8080),
-      .channel_sub = RGBA(0x0f2c4d80),
+      .channels = RGBA(0x194e8080),
+      .channels_sub = RGBA(0x0f2c4d80),
       .channel_group = RGBA(0x1a332d37),
       .channel_group_active = RGBA(0x216d5b67),
+      .channel = RGBA(0x12121299),
+      .channel_selected = RGBA(0x60c04044),
       .keyframe = RGBA(0xbfbfbfff),
       .keyframe_extreme = RGBA(0xe8b3ccff),
       .keyframe_breakdown = RGBA(0xb3dbe8ff),
@@ -442,7 +444,6 @@ const bTheme U_theme_default = {
     .header_text = RGBA(0xeeeeeeff),
     .header_text_hi = RGBA(0xffffffff),
     .shade1 = RGBA(0x96969600),
-    .shade2 = RGBA(0x12121264),
     .grid = RGBA(0x1a1a1aff),
     .vertex = RGBA(0x000000ff),
     .vertex_select = RGBA(0xff8500ff),
@@ -484,8 +485,6 @@ const bTheme U_theme_default = {
     .header_text = RGBA(0xeeeeeeff),
     .header_text_hi = RGBA(0xffffffff),
     .shade1 = RGBA(0xc0c0c000),
-    .shade2 = RGBA(0x1d1d1d99),
-    .hilite = RGBA(0x60c04044),
     .grid = RGBA(0x161616ff),
     .time_scrub_background = RGBA(0x1d1d1dff),
     .ds_ipoline = RGBA(0x94e575cc),
@@ -520,7 +519,6 @@ const bTheme U_theme_default = {
     .anim_non_active = RGBA(0x4d3b174d),
     .nla_tweaking = RGBA(0x4df31a4d),
     .nla_tweakdupli = RGBA(0xd90000ff),
-    .nla_track = RGBA(0x303030ff),
     .nla_transition = RGBA(0x1c2630ff),
     .nla_transition_sel = RGBA(0x2e75dbff),
     .nla_meta = RGBA(0x332642ff),
@@ -745,7 +743,6 @@ const bTheme U_theme_default = {
     .path_keyframe_after = RGBA(0xc4c4ffff),
     .gp_vertex_size = 1,
     .metadatatext = RGBA(0xffffffff),
-    .track_selected = RGBA(0xe7f7e74d),
   },
   .space_topbar = {
     .back = RGBA(0x18181800),