feat: support for only playing music from certain directories

[Bnyro]

closes #106

[SuhasDissa]

This works fine as long as the directory names are unique. IMO, It's better than nothing.

Try this to see what I mean,

• Add a few songs to the Music folder in the SD card.
• Add some more songs to the Music folder in the internal storage
• goto app settings and select the Music folder in SD card as the song location.

Expected behavior:

• App loads songs only from the Music folder in the SD card

Actual behavior:

• Songs are loaded from the both Music folders in internal storage and SD card

[Bnyro]

Songs are loaded from the both Music folders in internal storage and SD card

Yeah, I've considered this too while implementing - but the current way how we query music in combination with Android's DocumentContract API make it really hard to implement this in a better way.