BPM Detection (experimental) added & many Bugfixes

BPM Detection (start from Preferences Fragment) runs very slow. The
calculation results seems to be not really correctly.
Author of this solution: Peter Backx
(http://www.streamhead.com/easy-bpm-calculation-in-java/). THX to Peter

Bugfixes:
- Check for existing Folder before MusicFolderScanning (avoids crash if
the folder is not correct)

- Track Objects save and return the track duration in milliseconds and
as formatted string now (avoids a lot of crashes in the whole app,
which was depending with the wrong return format of track duration)
new UI icon elements
General information to PreferencesFrgmt added

Version information
prefixal version - ready for evaluation

All debug logs removed.

BPM-MetaDataInformation to Fallback switched (get bpm info from
filename)
Launcher Icon replaced
Merge branch 'dev'