This commit ensures that when a window is dragged across a monitor
boundary, the ownership of the window container will be transferred to
the target monitor's currently focused workspace.
In order to achieve this, a new WindowManagerEvent variant has been
added, MoveResizeStart, which will store an optional pending_move_op on
the WindowManager struct. This must be consumed at the beginning of the
handler for MoveResizeEnd.
This is necessary because as soon as the window is dragged across a
monitor boundary, an event is sent (and handled) to update the currently
focused monitor and workspace as the target monitor and workspace, and
we still need to have the information about the original monitor,
workspace and container in order to make comparisons and ultimately
remove the origin container to be able to transfer it.
fix#58
