Wait for window geometry before showing on wayland
Current state: When having 2 monitors of different size, first opening Yakuake on the smaller one and then on the bigger one, the window opens with the old scaling, i.e. it is cut off on the right.
This seems to be because the wayland surface is created before the async DBus call finishes and therefore the old size is used. With this PR, the DBus call was made synchronous.
Note: I think this would be succeeded by the LayerShell port as it doesn't depend on org.kde.PlasmaShell.StrutManager anymore