wayland: Fix Qt clients not being maximized initially

Currently, Qt clients send two maximize requests separated by the
initial commit. From spec's perspective, this is totally fine, the
client should receive two configure events with "maximized" state.

But because changeMaximize() in XdgToplevelClient and setMaximized()
operate on two different maximize modes, the second maximize request
will trick kwin into thinking that the client should be restored.