This should have enabled the current navigation drawer entry to be highlighted
by the call to setItemChecked() in selectItem(), but it still doesn't work.

Side-effect: some constant strings are now public

- Migrated I2PActivityBase and subclasses to Fragment
- Added new MainActivity with navigation drawer to handle Fragments
- Added MainFragment to navigation drawer

Code compiles, installs and runs. Known bugs created:
- Some buttons now fail (can't use an Intent with a Fragment)
  -> Migrate usage of Intents to Fragment transactions
- New install/upgrade dialogs are commented out
  -> Migrate old Dialogs to DialogFragments

The path in project.properties will require local overriding. I tried directly
referencing the SDK with ${sdk.dir} but the build fails with an absolute path
to a library.