Fix issues with multi-monitor setups in X11
The logic that tries to determine the available screen rect and position the main window accordingly, seems to be broken on X11 with multi-monitor setups: The window is only visible on the first screen and when scaling is active the absolute height is not considered properly, especially with scaling being active.
Fix this by unifying wayland/X11 with regard to determining the available screen size, both use the dbus API to determine the available work area. This avoids having to duplicate the calculation in yakuake. Unfortunately, on X11 the returned size needs to be corrected for scaling, compared to Wayland.
As a further note, we would not request the screen size on each window toggle, but install a dbus listener for the signal. In this case we would be able to adjust the window live and with fewer dbus calls.