Merge branch 'release/0.6.2.0' into develop
prepare for next development cycle
#39 correct the wording in download limit
#17 implement custom actions on back button
#10 Option to download images on demand
#46 App crashes when back button is used during downloading

quick and dirty solution for this NPE
release version 0.6.3.0
Merge branch 'release/0.6.3.0'