Applies drag resistance unidirectionally for horizontally/vertically
maximized windows, allowing them to be dragged without being untiled
immediately. When the distance of cursor movement orthogonal to the
maximized direction exceeds <resistance><unMaximizeThreshold>.
While dragging a horizontally/vertically maximized window, edge/region
snapping is disabled to prevent unintentional snapping and overlays.
This commit also includes some refactoring to simplify the logic.
...and unify region overlay and snap-to-edge overlay into overlay.c.
Snap-to-edge overlay is delayed for 500ms to prevent flickering when
the view is dragged from an output to another (demo in discussion labwc#1613).
This also fixes a bug that region overlay is not shown when a modifier
key is re-pressed.
When wanting to snap to a region when starting the move
operation with A-Left (or a similar mousebind which includes a
modifier), the modifier - or another one - must be pressed again.
Fixes#761
