Bugfixes for WorkoutXMLParser and FExListAdapter.

Bug in FExListAdapter:
If an exercise has no images the app crashed when a new training was started.

Bug in WorkoutXMLParser:
If an exercise had been deleted but was used in a workout the Workout could not be parsed/used anymore.
Moved classes to a different package.
Bugfix: added notification if there's no description of an exercise (in FExDetailFragment and ExerciseDetailFragment).

Before there was either a not-localized message "No description available" or an alert dialog with no text.