Port Client's decoration resize extend input window to XCB

Using Xcb::Window to manage the xcb_window_t and simplify the code - no
longer need to check whether the input_window is valid before calling e.g.
map, as that's handled in Xcb::Window.

One XLib usage for setting cursor is still present. This will be ported
once all the QCursor::handle() get removed.

REVIEW: 108771