Change the initial screen handling and add file load to OpenRocketViewer activity...

Change the initial screen handling and add file load to OpenRocketViewer activity.  The OpenRocketLoaderActivity is now used as a base class for the OpenRocketViewer and Main activities.  These activities now share the code used for loading ork files.  The OpenRocketLoaderActivity no longer is registered in the manifest because it is not a real activity any more.  The OpenRocketViewer activity now has a menu item for "load" to load a new model.  The Main activity uses finish() when it transfers control to the OpenRocketViewer thus removing it from the back stack.  Further, if the Main activity detects that the Application contains a rocket, it will automatically forward to the OpenRocketViewer.

git-svn-id: https://openrocket.svn.sourceforge.net/svnroot/openrocket/trunk@775 180e2498-e6e9-4542-8430-84ac67f01cd8