Add User Interface keymap (no items yet)

Needed to allow modal UI keymaps, but I'm sure we'll need this more often in future.
First item will be modal eyedropper keymap coming in a following commit.

release/scripts/modules/bpy_extras/keyconfig_utils.py

@@ -33,6 +33,10 @@ KM_HIERARCHY = [
     ('View2D', 'EMPTY', 'WINDOW', []),    # view 2d navigation (per region)
     ('View2D Buttons List', 'EMPTY', 'WINDOW', []),  # view 2d with buttons navigation
 
+    ('User Interface', 'EMPTY', 'WINDOW', [
+        # empty
+        ]),
+
     ('3D View', 'VIEW_3D', 'WINDOW', [  # view 3d navigation and generic stuff (select, transform)
         ('Object Mode', 'EMPTY', 'WINDOW', []),
         ('Mesh', 'EMPTY', 'WINDOW', []),