A WebActivity has been added back to encapsulate the web browser functionality,
allow the nav drawer items to be launched and enable other fixes.

Refreshing is now disabled in PeersFragment and WebFragment, but both of them
will eventually be removed.

All page logic is in the Fragments. Activities don't need the convenience
methods.

zzz and sponge pointed out that the router needs to be managed by whatever
Activity happens to be active (in case an automatic start is required). This
commit reverses the initial migration of I2PActivityBase to a Fragment (code is
broken now).

There are a lot of changes here:
- The main_content FrameLayout is now main_fragment (more logical name).
- TunnelEntry now stores a Context instead of a TunnelEntryLoader (so
  it can be instantiated inside the detail Fragment).
- The activity_navdrawer layout is now an alias to a one-pane or two-pane
  layout depending on screen size.
- Subclasses of I2PActivityBase can now override canUseTwoPanes() to enable
  and use the two-pane layout (for devices that support it).

Broken: cannot find net.i2p.i2ptunnel.I2PTunnel
(but it can when launching the router)

Temporary; will eventually be in an about menu in settings

The navigation drawer has been (mostly) migrated to link to Activities instead
of Fragments. Only Activities can be searched via the inbuild Android search
function, which uses Intents.

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