reenable sorting by sender in messagelist

This was disabled in faa666394ce0bf9e4239869ee194991d2f3308f1
because it isn't possible to extract the name of the android
contact in the 'ORDER BY...' clause when querying the database.
Instead it simply sorts by the email address.

This may cause the same contact to appear multiple times in
the list, if they have multiple email addresses assigned.

But in most cases this is good enough and surely better than
not having the option to sort by sender at all.

Desktop mail clients such as Thunderbird also simply use the
sender email information when sorting the column.

This also adds a SenderComparator for usage in the MergeCursor.
Use themed context when creating EmailAddressAdapter

Previously the activity theme was used to display the auto-complete
suggestions. This lead to unreadable text when the activity theme was
different from the "composer theme".
Fix condition on when to show "check mail" in POP3 accounts
Avoid NullPointerException in SubjectComparator
Don't create folder -NONE- when excluding special folders from a search
Get rid of accidentally created "-NONE-" folder
Don't crash when being started by an old unread widget intent
Fix NPE introduced in 340d6e0522b7cb03349e6f60ee169e8b7452fbcb
Fix NPE when fragment detached from activity
Avoid NullPointerException
Avoid IllegalStateException

Reported for Kaiten:
java.lang.IllegalStateException: Can not perform this action after
onSaveInstanceState
...
at android.support.v4.app.DialogFragment.dismiss
Add "Mark all as read" action to message list
Show "check mail" action when displaying Unified Inbox etc.
Fix unread count in notifications (again)
Update German translation
Make status display less awkward when checking mail from Unified Inbox

We really need to fix the ActivityListener madness :(
simplify the messagelist chips based on lots of user screaming. default checkboxes to being on.
Update strings.xml
Don't show a disabled preference if there is nothing a user can do to enable it.
Issue 4503: Auto-fit messages option

Create a preference option to automatically shrink messages
to fit the screen width (default setting is "shrink").