Adds states and request methods to determine whether a window can be put into
move or resize modes, and request move and resize modes, respectively.
request* naming pre-discussed with Martin. I chose to add *Mode to be more
explicit as well as avoid namespace conflicts with possible other Move/Resize
methods in the future.
Since these are not toggleable states, there is no requestToggle* methods.
Protocol version is not bumped (also pre-discussed with Martin) since we have
pending changes already bumping to 3.
Depends on D1417.