show() + activateWindow() might not activate the window on X11

If you call show() and then activateWindow() right away this might result in a race and the window will not be activated because the window might not be viewable by the time when we call XSetInputFocus, so activateWindow() will not do anything.

We should probably remember that the user asked to activate the window and do that when we get MapNotify event, i.e. when the window becomes viewable.