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

...s are in use or task manager is used. r=alive
I lied, I am a genius and sheets-gesture-end from StackManager works just fine and is always called as expected. Major clean-up done because of that. 

I also changed sheetstransitionstart to sheets-gesture-begin and updated all relevant code that used to use the old name sheetstransitionstart.