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:

![image](/uploads/a8a63bc204f9ae9ef7b6831ededdd78f/image.png)

after, just open:

![image](/uploads/aa26abd2339e1d3dcb9549751e633414/image.png)