gdk/toplevel: Negotiate surface size via a compute-size signal

gdk/toplevel: Negotiate surface size via a compute-size signal

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.