APRSdroid activities and inheritance

LoadingListActivity (Android ListActivity) -- helper to show progress spinner while loading
- ConversationsActivity -- shows all APRS message conversations
- MainListActivity -- helper with start/stop action buttons at the bottom
  - HubActivity -- list of stations sorted by distance
  - LogActivity -- list of all packets sent and received
- StationHelper -- helper for per-station menus
  - MessageActivity -- conversation with a callsign
  - StationActivity -- details of a callsign, list of all SSIDs, list of all packets
MapAct (MapsForge MapActivity) -- offline map
GoogleMapAct (Android Activity) -- online map
PrefsAct (Android PreferenceActivity) -- central preferences
PrefSymbolAct (Android Activity) -- chooser for APRS symbol
ProfileImportActivity (Android Activity) -- JSON profile file import
KeyfileImportActivity (Android Activity) -- SSL key (.p12) file import