Revert "wayland: remove outdated comment"

This reverts commit d75cfef49c271e43317666451f17c55e6747e564.
Revert "wayland: remove moving window by grabbing"

This reverts commit 3308bc2bc919cb8fadddad04e48b94d4e9324a34.
wayland: use mp_input_test_dragging

Instead of removing dragging we now test if it we should drag the window or
not. Because if the OSC shows up we can not drag the window because that would
cause mouse events that makes the OSC disappear.