Every display needs a top window (topwin). The first window created for a
display automatically becomes its topwin. The topwin can be changed later
with the mbtk_display_set_topwin() call.

If the current topwin is closed, a new topwin is elected. It's better
if the application closes the display after closing the topwin or
explicitly sets a new topwin before closing the old topwin.

Where topwin matters:
 - non-window events (e.g. stdin or fdwatch) are reported for the topwin
 - some backends may depend on performing per display actions on a window,
   e.g. glfw mouse cursor or clipboard calls; these will use the topwin
 - in some cases modality may require naming a main window of the application