zxtune-android: cleanup contexts usage. Fix possible crashes.
zxtune-android: fixed cache for zxart.ee catalogue.
zxtune-android: support compos in zxart.ee source.
zxtune-android: fix db update for modland and zxtunes sources.
zxtune-android: get rid of VfsDir.find method.
zxtune-android: introduce and use app.zxtune.fs.StubObject.
zxtune-android: refactor code in zxart.ee source.
zxtune-android: deutche translation of zxart.ee texts update.
zxtune-android: use internal author/title for additional information in zxart.ee catalogue.