Delete shortcuts so it works when we load a new config. Add shortcut for watershed

[dwlsalmeida]

Unless we hook a delete call for the shortcut when LabelInfo objects go out of scope, we will not be able to add a new shortcut for the same QKeySequence.This is problematic in the event we want to load another config file: shortcuts simply will not work in this case. This quick change fixes this.

I also took the opportunity to delete 'item', since it is a pointer type acquired with new that is not destroyed anywhere (I guessed by briefly looking at the source code, since I am not that familiar with Qt)

Lastly, I added a shortcut for the run watershed button while I was at it.