Clutter 1.15.2 Improves the Wayland Backend

Compared with the stable Clutter 1.14 release, this development version introduces improvements and updates for the Wayland backend, documentation updates by cleaning up the API reference and improving the comments in the inlined example code (the cookbook is also included in the source archive). Moreover, Clutter 1.15.2 improves short circuiting and state tracking, allowing ClutterActor to track unmapped actors and clones more aggressively, and to reduce the amount of work required when updating the scene graph. ClutterFlowLayout:snap-to-grid has been added to Clutter 1.15.2, which allows users to ask the layout manager to not align the actors to grid, and installation of conformance tests into well-known locations is now allowed. Last but not least, the ClutterText::cursor-event has been replaced by ClutterText::cursor-changed, and ClutterGeometry, ClutterActor::realize and ClutterActor::unrealize have been deprecated, along with their virtual functions.