View tracks with any installed viewer, also if Google Earth is not installed about gpslogger

[GrazianoCapelli]

Code updated in commit 5ffec20.

Now the behavior is the following:

1. No viewers installed: the menuitem is hidden;
2. 1 viewer installed: the menu "View in App_Name" is shown (where "App_Name" is the name of the installed viewer);
3. Many viewers installed, default app set: the menu "View in App_Name" is shown (where "App_Name" is the name of the installed viewer);
4. Many viewers installed, no default app set: the menu "View in ..." is shown. The user will choose between them (like the "Share" menu) using a selector;

The code has been tested, and will be tested again, in order to be ready for the next app update.

I'm thinking about the point 3: Maybe is better to force users to choose between apps also if a default one is set into the system. Is more annoying, but many users don't know how to revert a default app from Android settings.

[GrazianoCapelli]

Next app update will be published with the behaviour described above.
We check the users feedback and, in case, we'll evaluate to refine or modify this behaviour.

[JLJu]

User-driven. That's fine!