Dialog: Split wayland parts into new class

The PlasmaShell wayland parts are split out of Dialog, allowing for
re-use in upcoming replacements.

In addition we move away from KWayland in favour of generated classes
and QtWaylandClientExtension. We also change the lifespan of the
user-facing class to match the Window rather than the plasma_surface.

This allows a caller to call setPanelBehavior once and sending it
multiple times as the underlying shell surface is recreated should
happen automatically.

This also should fix ordering applying initial plasma_surface
properties before the first commit.

Whilst Dialog could now be cleaned up further, this commit does not do
so as it is being phased out.

BUG: 428147