Allow any window to display a resize grip, in the south-east or
south-west corner, depending on text direction. This is implemented
as a shaped window that gets overlayed on top of whatever content
is there. We add api that allows widgets to avoid the resize grip,
if desired.

The ::has-resize-grip property controls if a window may display
a resize grip. It will only be displayed if the window is resizable
and not maximized.

The size and visual appearance of the resize grip is under theme
control, using the resize-grip-width/height style properties and
the paint_resize_grip style function.