1. No longer fallback to use move(), since all known platforms
support startSystemMove(), and it's unlikely going to work
if it's not supported on a specific platform.
2. Fix titlebar move check missing `window()->`.
This amends a10bbb4e8d
This better matches titlebar behavior of Windows and KDE, and probably a lot more other environments, it also allows for easier unmaximizing because the current titlebar has no dedicated maximize/restore button.
Note: This is somewhat problematic on Windows, thus we only apply the change to other platforms for now.
