Added a .json file that contains the ExerciseTags.

The ExerciseTags will now be loaded from this .json file.
Bugfix FExDetailFragment: Fixed editing TrainingEntry.
Changed icon colors from white to grey.

On older Android systems the menu background can be white and the icons were invisible.
Some litte layout improvments. Removed the license text, added menu item with dialog instead.

Also improved the dialog for showing other training entries. If there are no other entries there will be a Toast instead of the dialog. The current training entry will no longer be shown in the list of other training entries.