interactive: Make window snapping with mouse more intuitive

interactive: Make window snapping with mouse more intuitive

1. Prevent window snapping triggered by mouse from moving the window
 into the adjacent output.
2. Make the coordinates used to check whether window snapping is
 triggered relative to the output the cursor is at, not the output the
 view is belonging to. This allows users to grab a tiled window and move
 it into another output or tile it again in another output in a single
 drag.