[window] Allow user-set sizes before show
[window] Allow user-set sizes before show

If the user has manually set a size on the window before showing it,
honour it (but still enforce the minimum size). Also make sure to check
that the minimum size is valid.

pwithnall/mx,pwithnall/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,jonnylamb/mx