Add a virtual_desktop_changable state to the window protocol.

Summary:
Analogous to NET::ActionChangeDesktop.

KWindowInfo::actionSupported(NET::ActionChangeDesktop) is hardcoded
to return true in kwin, but that's not how it should be; as this will
be fixed later the Wayland protocol needs this state bit as well for
parity.

Test Plan: PlasmaWindowModel test is extended.

Reviewers: graesslin

Subscribers: plasma-devel

Projects: #plasma

Differential Revision: https://phabricator.kde.org/D1448