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

...s are in use or task manager is used. r=alive
Yeah... I totally see what you mean. 

Here's what I did -- I make AppWindowManager sort out that whole mess of batching vs peeking only and it sends 'sheetsgesturebegin' and 'sheetsgestureend' when appropriate. The AppWindow now simply checks to see if it's active or not. 

I would've liked to use 'sheets-gesture-begin' and 'sheets-gesture-end' but we don't munge the event names before calling handlers in AppWindow so we could have 'sheetsgesture[begin|end]' or 'sheets_gesture_[begin|end]'. I made it a compound word for now since that's what we did with the others (except kill_suspended...)