Merges the desktop grid and overview effects together in a new three-state one;
you can switch between them with a certain shortcut or gesture, and you can also
still access either the desktop grid or overview directly.
Default shortcuts are also updated to be Meta+G for Grid, Meta+W for Overview,
Meta+Tab to switch between the three states and Meta+Shift+Tab to cycle in the
opposite direction.
BUG: 474044
BUG: 460661
BUG: 460774
BUG: 456572
BUG: 449601
BUG: 450262
BUG: 449801
BUG: 461510
BUG: 463886
BUG: 459754
BUG: 459749
BUG: 459748
BUG: 459467
FIXED-IN: 6.0
