Interface: New region type 'Footer', used by text editor

* It can be hidden by dragging it up/down.
* It can be at the top or bottom, independent of the header.
* It uses the color theme from the header.
* It does not change its color, when the area becomes active.

Currently, it is used in the text editor to display the file path.

Differential Revision: https://developer.blender.org/D4601

release/scripts/modules/bl_keymap_utils/keymap_hierarchy.py

@@ -55,6 +55,7 @@ _km_hierarchy = [
     ('Screen', 'EMPTY', 'WINDOW', [     # full screen, undo, screenshot
         ('Screen Editing', 'EMPTY', 'WINDOW', []),    # re-sizing, action corners
         ('Header', 'EMPTY', 'WINDOW', []),            # header stuff (per region)
+        ('Footer', 'EMPTY', 'WINDOW', []),            # footer stuff (per region)
     ]),
 
     ('View2D', 'EMPTY', 'WINDOW', []),    # view 2d navigation (per region)