[File] Add Filesystem class for better testing

[bugwelle]

[bugwelle]

The idea was to pass an explicit file system object to each class that handles files. However, I now think that having global functions or at least a singleton isn't bad in this case. Simply because we're not a library, we're an app. Having a central place that handles files is useful here.

[bugwelle]

Further ideas:

• Should there be a central object that others can connect to?
• Should we leave it at the current installFileSystemLog state? We can move to the before mentioned object quickly by adding it to FileSystemLog.cpp

[bugwelle]

Discarding. See https://github.com/Komet/MediaElch/issues/847#issuecomment-1908709436