Add snap to window edge framework

Adds functions for calculation of distances between window edges, as
well as for window growing and shrinking.

All calculations are based on the "pending" geometry.

Ignored from snapping:

 - views that do not share the same output
 - minimized views
 - maximized views
 - views that are neither:
   - part of the current workspace
   - part of the always-on-top tree

src/meson.build

@@ -20,6 +20,7 @@ labwc_sources = files(
   'seat.c',
   'server.c',
   'session-lock.c',
+  'snap.c',
   'touch.c',
   'theme.c',
   'view.c',