Better cover finding
Reason for the change
Build cover file search strings from file name + extension
So we don't have to duplicate the file name string for each extension. Also fixes a bug where we forgot to search for the "jpeg" extension.
If there is a single image in an album folder, use that as the cover file
Might as well use it
Prioritise exact string match when searching for cover files
Sometimes a user might have files such as "Case Cover Back Inner.jpg" and "cover.jpg" in the same music folder. Previously the cover finder would search filter these alphabetically using the glob "*[Cc]over*.jpg" and use the first result as the cover. For this example, we would end up using "Case Cover Back Inner.jpg" which is clearly not the intended cover image.
Now, we prioritise direct matches for "[Cc]over.jpg", "[Cc]over.jpeg" and "[Cc]over.png" before we check the globs. So for our example, the correct cover file would be chosen.
Use the first alphabetically found image file for cover image as a last resort
Might as well show something if none of the previous attempts to find a cover image were successful
Test plan
Updated filescannertest.cpp