Two tiling drag fixes #43

In `handle_motion_postthreshold()`, not all cases of dragging a
container onto itself will be caught. This commit adds an explicit check
to deny a move operation if the container being dragged is the same as
the container under the mouse cursor. If this case isn't caught, the
container being dragged will disappear from the layout.

Previously, in commit 4d4611f8 ("seatop_move_tiling: Special case
handling of tabbed containers"), `L_TABBED` containers were handled
specially, by traversing all the ancestors of the container being
hovered looking for the first `L_TABBED` container.
Originally, I thought this traversal was necessary since the mouse
cursor might be over an application window in a tabbed container and we
would want the parent container of that window, so I copied the
traversal logic from later in the same function ("// Traverse the
ancestors"). However, after debugging the issue in the previous commit
("seatop_move_tiling: Don't allow moving a container onto itself") and
looking at this more closely it appears that `node_at_coords()` is
already doing the work to find the parent container of the window under
the mouse cursor.
(In other words, if the cursor is hovering over an application window
inside an `L_TABBED` container, I believe `node_at_coords()` will return
the parent `L_TABBED` container, not the node of the application
window.)
This commit simplifies the handling of tabbed containers in
`handle_motion_postthreshold()` by only considering the hovered
`L_TABBED` container rather than traversing all of the hovered node's
ancestors.