action: allow SnapToEdge to combine two cardinal directions

action: allow SnapToEdge to combine two cardinal directions

This patch adds `combine` argument to (Toggle)SnapToEdge actions.
This allows to snap a window to e.g. up-left by running two actions:
- `<action name="SnapToEdge" direction="left" combine="yes" />`
- `<action name="SnapToEdge" direction="up" combine="yes" />`

Then running `<action name="SnapToEdge" direction="down" combine="yes" />`
snaps it to left again. This behavior is almost the same as KWin, except
that snapping a up-right-tiled window to right doesn't move it to the
right-adjacent output, but makes it right-tiled first.