A window is a rectangular area registered as a window in the window manager.
The x;y position of a window is the screen coordinate of the top-left corner
of the window (first pixel of the content).

A subwindow is a rectangular area within a window. The x;y position of a
subwindow is the top-left physical pixel of the subwindow in parent window
or subwindow coordinates. A subwindow can host further subwindows.

The content of a subwindow may be larger than the subwindow and may be shifted
(scrolled). The top-left pixel of the content, in content-coordnates, is
the subwindow's ->content_x0 and ->content_y0. These fields are always zero
for a window.

Both window and subwindow are represented by mbtk_window_t. A subwindow's
top-parent is its final parent window in the hirerchy, which is the root of
the subwindow tree and is always a real window.

A subwindow is normally used for two reasons:
 - content scrolling (e.g. the scrolled widget, or the textedit widget)
 - separate drawing context (e.g. the canvas widget)

Event reporting:
 - keyboard events are always reported for the top-parent window
 - mouse motion and mouse button press/release and mouse scroll is always
   reported for the innermost subwindow (or the top-parent if no subwindow is
   found at the given coords), in content coordinates.
 - window enter/leave/move/resize events are not delivered for subwindows,
   only for windows

Focus widget: only the top-parent window has a focus widget.