When being fullscreen, and wanting to unfullscreen but not caring about
whether to go unmaximized or maximized (as this information is lost), if
the GdkToplevelLayout represents the full intended state, we won't be
able to do the right thing.
To avoid this issue, make the GdkToplevelLayout API intend based, where
if one e.g. doesn't call gdk_toplevel_set_maximized() with anything, the
backend will not attempt to change the maximized state.
This means we can also remove the old 'initially_maximized' and
'initially_fullscreen' fields from the private GtkWindow struct, as we
only deal with intents now.
GTK will not up front know how to correctly calculate a size, since it
will not be able to reliably predict the constraints that may exist
where it will be mapped.
Thus, to handle this, calculate the size of the toplevel by having GDK
emitting a signal called 'compute-size' that will contain information
needed for computing a toplevel window size.
This signal may be emitted at any time, e.g. during
gdk_toplevel_present(), or spontaneously if constraints change.
This also drops the max size from the toplevel layout, while moving the
min size from the toplevel layout struct to the struct passed via the
signal,
This needs changes to a test case where we make sure we process
GDK_CONFIGURE etc, which means we also needs to show the window and
process all pending events in the test-focus-chain test case.
