This fixes the following bugs:
- Sort category is forgotten after lock/unlock
- The sort mode is not respected for new entries
I got a little carried away while working on this patch and also included the
following other enhancements:
- Simplify the SortCategory, Theme and ViewMode enums
- Simplify usage of string resources
- Don't call notifyDataSetChanged and runLayoutAnimation unnecessarily
This adds an option to automatically lock the app when:
* The back button is pressed
* The device is locked
It's the first step towards implementing #7