Compositor: Fix refresh rate

Wayland output defines refresh rate in millihertz (int) while Qt uses Hz
(float). Make sure all integer APIs are using millihertz, and covert Hz
values to mHz when passing from Qt.

Change-Id: I5886f5618680d99db6fc106dd7b8998b00a2face
[qtwayland] Always update window on commit
[qtwayland] Respect windowState during creation

QWaylandWindow ignored windowState set before creation, making
showMaximized, showFullScreen, (showMinimized) ineffective.

Also remove the forced visibility initialization at QWaylandSurfaceItem init.
[qtwayland] Remove unused member
[qtwayland] Check for invalid size before calling configure

If window size was not set, configure was called with size -1,-1
which was considered non-empty by QWaylandWindowConfigure.