x11: Deal with window borders better. - Cache the _NET_FRAME_EXTENTS data locally, so we don't have to query the X server for them (instead, we update our cached data when PropertyNotify events alert us to a change). - Use our cached extents for X11_GetWindowBordersSize(), so it's a fast call. - Window position was meant to refer to the client area, not the window decorations, so adjust appropriately when getting/setting the position.