UI: Use flat colors for NLA strip drawing

The NLA editor is in need of a design overhaul, hopefully for 3.1 or 3.2.

This should be a project on itself, however, the worst offender currently is the use of
gradients on strips. Something that can be fixed easily.

{F11390293, size=full, loop, autoplay}

A simple replace of `UI_draw_roundbox_shade_x` for `UI_draw_roundbox_4fv` brings strips
in line with how other areas are drawn.

This patch also:
* Remove embossed lines around active action channel.
* Highlight the strip while being moved.

This patch does not include any theme changes. This will be tackled separately.

Reviewed By: HooglyBoogly

Differential Revision: https://developer.blender.org/D12968

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