QML: fixes for switch between carded, maximized and fullscreen

 * keep the corner shader active until the window is actually maximized
 * move the logic for detecting which kind of swipe happened into LunaGestureArea
 * if a window is fullscreen or maximized, change to card mode before showing the app launcher
 * fix interactions with cards

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>