Add support for dashclock lockscreen widget.

This commit includes support for popular dashclock lockscreen widget.
Dashclock widget playstore url:
https://play.google.com/store/apps/details?id=net.nurik.roman.dashclock
When the above dashclock widget is installed, users of K9 will be able
to see unread message count on the lockscreen. It also includes a
setting activity to choose email accounts to show(only email/
description and unread count) in expanded view of dashclock extension