Anim: Improve readability of channel colors in channel list

Draw anim channel colors as a little rectangle in the channel list,
instead of taking over the entire channel background. This keeps the
channel names readable, regardless of the channel colors.

Channel colors are typically set via the bone colors, and since those
are chosen for visual contrast in the 3D Viewport, they aren't
guaranteed to also be a suitable background color for the channel list.
Because of this, it's no longer used as such.

Design task: #69059

@@ -4777,0 +4714,4 @@
+      /* Little channel color rectangle. */
+      if (acf_show_channel_colors()) {
+        const float rect_width = 0.5f * ICON_WIDTH;
+        const float rect_margin = 2.0f * U.ui_scale;

@@ -5610,0 +5604,4 @@
+      /* Little channel color rectangle. */
+      const bool show_group_colors = acf_show_channel_colors();
+      if (show_group_colors) {
+        const float rect_width = 0.5f * ICON_WIDTH;

@@ -228,1 +179,3 @@
-  }
+  /* FIXME: what happens when the indentation is 1 greater than what it should be
+   * (due to grouping)? */
+  int colOfs = 10 - 10 * indent;