UI: Initial persistent floating redo panel

This is the first step to moving redo out of the top-bar, see: T55039

- Support for floating panels in the ScrArea has been added.
- A new region type RGN_TYPE_HUD is used for floating settings input.
- Only one HUD should be visible at a time.
- Currently HUD regions are only visible
  for 3D view and image space types.
- There are currently some remaining glitches with hide/show
  that need further investigation.

source/blender/editors/include/UI_interface.h

@@ -830,6 +830,7 @@ struct Panel *UI_panel_begin(struct ScrArea *sa, struct ARegion *ar, struct List
 void UI_panel_end(uiBlock *block, int width, int height);
 void UI_panels_scale(struct ARegion *ar, float new_width);
 void UI_panel_label_offset(struct uiBlock *block, int *x, int *y);
+int UI_panel_size_y(const struct Panel *pa);
 
 bool                       UI_panel_category_is_visible(struct ARegion *ar);
 void                       UI_panel_category_add(struct ARegion *ar, const char *name);