xwm: Don't change frame button state to _HOVER when pressed down

Previously, clicking down on a frame button and moving the mouse
cursor while the button is pressed down, would reset the click-down
event and, consequently, make nothing happen when the mouse button
was released. This fix makes sure that this doesn't happen, by only
changing the button state to FRAME_BUTTON_HOVER if the button is
*not* already pressed down.