Added detail page for tunnels and a two-pane layout for large screens

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).