bug 1044125 - Only capture screenshot and show overlay when edge gesture...

...s are in use or task manager is used. r=alive
Not a big fan... the added bit of state on each appWindow will probably drive me _batchsheetcrazy_. (sorry, I had to) :wink: 

More seriously
* the stack manager should dispatch real `CustomEvents` instead of calling the `AppWindowManager` directly
* if we absolutely have to the `AppWindowManager` should handle the state and broadcast or not the `sheetstransitionend` accordingly
* but, even better, looks like with the way you implemented the `sheetsbatchend` we're always getting one, so we can remove the `sheetstransitionend` altogether and stay with one code patch which will be much better.

To make things cleared we should rename `sheetstransitionstart` in `sheets-gesture-begin` and `sheetsbatchend` in `sheets-gesture-end` it will have a nice symmetry.