windowflags: Lock fixed size to current size instead of 300x300

The latter would allow one final resize that would immediately
jump to 300x300.

Change-Id: I566e5e9dc1fb07f748f528f002166a8438344173
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>