[Suggestion] Expand extracting cover art images to artist images and to detect other common file names

[Olf0]

Extend the code introduced by PR #75 (primarily in src/datareader.cpp) to:

1. Extract (detect & copy) artist images, see last paragraph of this #75 (comment) for some details.
2. Detect other common file names, e.g. front, if they really are "common" (please check first!).
   Currently, only cover and folder are detected.

P.S.: Originally this was the second and half of the third point of this list #75 (comment).