Indicate changed resource files

[RumovZ]

@bohning, I've reimplemented your feature from #494.
I've taken your icons for the system theme, and chose new Material icons for the dark theme.

Two caveats:

• Sync status is cached in the Python layer, so updates are only detected at startup, or when the song is reloaded, like after download or song folder change.
• Scrolling local songs initially is noticeably slower.