UI: Add frame range theme options
Allow changing theme colors for the start/end frame range
overlay, similar to changing the preview range colors.
Affects dopesheet, graph, timeline, movie clip (graph/dopesheet)
and sequencer editor.
I tried working on a high contrast theme for Blender, which uses a fully black (#000000) background extensively.
You could not see the scene's start/end frame ranges, due to the hardcoded shading values.
