Add states and request methods for move and resize.

Summary:
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.

Test Plan: PlasmaWindowModel test extended.

Reviewers: graesslin

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1432