When closing the panel from (eg.) a tap on the backdrop, reset and turn off integrated panel scrolling

[willbryant]

Otherwise it's impossible to drag the panel up again by the header, because although we receive the drag events on the listener, _scrollingEnabled is set to true, so the slide handler ignores the event.

[willbryant]

Rebased to master and fixed merge conflict with the dart format commit.

[GabrielBB]

@willbryant How did you achieve closing the panel with a tap in the backdrop? Should I use defaultPanelState ?

[willbryant]

I don't have any references to defaultPanelState and I don't remember/see any code in my app to handle backdrop taps, I think it is the default behavior?