Previously, TLW geometry was implicitly defined as just its position,
size and the maximized/iconized state by wxPersistentTLW code. This
already wasn't enough for wxGTK which added the decoration sizes to the
geometry being saved/restored, but this had to be done using conditional
compilation, which was not ideal. And it didn't allow using an entirely
different geometry representation as will be done for wxMSW soon.
Change the code to use wxTLWGeometry class defining the geometry, as
used by the current port, explicitly and move wxPersistentTLW logic into
it, as wxPersistentXXX classes are supposed to be very simple, which
wasn't really the case.
Also provide public SaveGeometry() and RestoreToGeometry() methods in
wxTopLevelWindow, which can be useful even to people not using
wxPersistentTLW for whatever reason.
There should be no changes in behaviour so far.
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