* Internal windows

** Problem

PLWM is increasingly creating windows: modewindow, menu, and the
upcoming frames support will also have windows of a sort.

There should be a basic class to use for this, which provides event
handling, and if needed, masquerading the internal window as a client
window so that the ordinary functions for handling things like focus,
borders, and resizing work transparently.

** Solution

Define a basic class, or maybe two: one for simple internal windows
and one for clientish windows.

** Followup

Most of this has been done, although internal windows still can't
behave as clients all the time.  Some mixins, e.g. the BorderClient
could be abstracted to allow internal windows to have the same kind of
borders as client windows.


