Rework grid plugin

[ammen99]

The grid plugin is currently a mess. It needs a huge rework, and it must support the following things:

1. Support animating more than 1 window at a time
2. Restore the geometry on set_maximized(false) when maximized to any of the slots
3. Change tiled windows' geometry on the reserved-workarea signal

Another possibility is to expand the tiling plugin to tile only specific views and integrate grid and tile. However, this will require much more work to the unfinished tile plugin and possibly goes outside of its scope.

[ammen99]

Resolved in master.