UI: improve mesh edge highlighting

Changes to edit mode mesh overlays, use hue shift instead of color
fading/darkening for selection mode visual differentiation, and some
theme changes to improve the display of mesh edges and faces with good
selection visibility.

- Removed "edge" toggle from edit mode overlays panel.
- No longer halves the edge and face alpha depending on selection mode.
  Half the face alpha in wire-frame mode. For better visibility on most
  themes.

Ref !111431

scripts/startup/bl_ui/space_view3d.py

@@ -7055,9 +7055,6 @@ class VIEW3D_PT_overlay_edit_mesh(Panel):
 
         split = col.split()
 
-        sub = split.column()
-        sub.active = is_any_solid_shading
-        sub.prop(overlay, "show_edges", text="Edges")
         sub = split.column()
         sub.prop(overlay, "show_faces", text="Faces")
         sub = split.column()