Fix Qt6 Overlaysheet regressions and improve behavior

Improves the behavior and simplifies the code. Pointerhandlers didn't really work for the problem in particular, a MouseArea outside for the click to dismiss and one inside for the drag away to dismiss

work much better without glitches, now:

• there are no new items, only 2 ported from Item to MouseArea
• the code is actually much simpler
• the slide to dismiss works only when dragging actually the sheet, not by dragging empty space which was weird and very unreliable
• click to dismiss only triggers if the cursor/finger was not dragged around
• it doesn't move up/down anymore when scrolling the content for the first pixels, the behavior was very glaring

before, just open:

after, just open: