Cover Files Extraction Branch

[Alexander Stippich]

Hello everyone,

I've pushed a branch called cover_manager.
It contains a working solution for the extraction of cover images embedded in 
the files, but also does some more stuff. So here is a list:
-reorder all platform-agnostic scanning files to a single folder
-factors out the current code for cover searching into own class called cover 
manager
-add tests and simple benchmarks
-optimizations for cover searching
-extract the cover art from music files
-resize when image is too large (512x512 current maximum)
-save all covers to standard data location, e.g. the same location where the 
database is located

Future things I would like to look at:
-add a QQuickImageProvider for the cover manager (opinions?)
-add functions to create artist pictures from the album covers (like in 
https://community.kde.org/KDE_Visual_Design_Group/Music_Player)
-make maximum cover size configurable

I would appreciate any feedback. I _think_ currently it is necessary to delete 
the old database in order to get the new images, but this is an issue 
unrelated to this branch.

Best regards,
Alexander