XdgPopupWindow: Reposition for non-reactive positioners

Ensures that e.g. context menus move about with their parents when they
get moved around.

However, as per spec don't re-constrain the window when its positioner
is non-reactive. This change calculates the offset from its parent window
once initially and places the window relative to that whenever the parent
moves.

Only when the positioner is reactive, will it recalculate the placement fully.

BUG: 461994