Blender Git Loki

Fix T82417: Panels draw below others while animating after drag

This adds a new runtime flag for panels that is set during the entire
drag and animation operation. The flag is set recursively so that
sub-panels know to draw on top too.

Note that this also replaces most of the fixes in 1960b8a361ee and
8e08d80e52d6 (D7462) with a more "built-in" solution.