Add the necessary colors and/or alpha components to the theme instead.
Also switch the background for ordinary channels to use the likely
intended theme option, instead of the window background color.
The general rule is that the channel color is drawn full strength in the
channel list on the left, and with alpha in the actual key frame area on
the right. This alpha is also reused with bone group colors.
Reviewers: brecht, billreynish
Differential Revision: https://developer.blender.org/D3813
For now we have categories collection, object, object data, modifiers &
constraints, and shading. The icons can be categorized by adding e.g.
DEF_ICON_OBJECT() in UI_icons.h.
Light themes will need to be updated to use darker colors to keep icons
visible in the outliner.
It's more convenient to keep version patching in the same place,
this also splits out some function calls from version patching
and supports updating UserDef structs besides the 'U'l global.
