Blender Git Loki

UI Code Quality: Refactor panel drawing function

The existing panel drawing function was a bit convoluted with dependent
conditions in different scopes, redundant and unecessary computations,
and un-helpful naming.

This commit separates the function into two parts, the backdrop and the
widgets. It also improves naming and uses const where possible, and in
general cleans up the code.

There are some slight visual changes, mostly with the placement of the
drag icon, which moves a bit downward to be centered with the triangle
icon. The black rectangle displayed while dragging is also removed.