Make PhocXdgShell's struct members private

As hinted in !421 (merged) there wasn't that much left to do. I kept it minimal (no function reindents when code only moved, no s/phoc_surface/self/ renames) but it's a major step in isolating things better.

PhocXwayland should be able to follow the same pattern. PhocView will be more work but we're getting there.

This contains !421 (merged) so I'm happy to rebase in case that lands first.

Prereq for !423 (merged)