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 ldpi resources
Remove unused resources and merge resource folders
Fix typo in attribute name
Remove commented-out code
Whitespace fixes
Merge pull request #492 from k9mail/remove_unused_resources

Resources cleanup
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