Clean up the code a bit
First very basic version of a split screen for message list + view
Add marker in message list for currently opened message
Add setting to configure the splitview mode
Add 1px divider between message list and message view in split view
Add hotkeys from MessageView to MessageList
Add message view menu items to MessageList
Go back to message list when using the back button in message view
Make sure swipe gestures only work on the message list
Fix "return to list after delete"
Get rid of MessageView activity
Re-order global preferences -> display.

Put folder list options between account list options and message list
options, so the order matches the order the respective activities appear
on screen.
Improve Account settings -> notifications layout.

It makes no sense to display ringtone and vibration as children, but not
doing that for the vibration pattern and length. As double indention is
not available, un-indent ringtone and vibration and indent the vibration
parameters instead.
Update and improve German translation.
Fix back button behavior
Set MessageList title in action bar depending on display mode
Use theme-specific color for message list divider
Remove unused layouts
Extract string to strings.xml
Change default value of split-screen setting