Configure mail notifications by folder class

In response to Issue 1794,
- add a configuration option in the account preferences to show
  notifications only for 1st/2nd/etc class folders
- add an option in the folder preferences to set the notification class
  as 1st, 2nd or inherited from the folder's push class

The default behaviour remains unchanged.
Remove commented-out code
Whitespace fixes
Simplify database upgrade when adding 'notify_class' to 'folders'
Remove getNotifyClass() from Folder
Increase settings version in preparation of merge into master
Merge pull request #485 from haselwarter

Conflicts:
	src/com/fsck/k9/preferences/Settings.java