Add a dedicated service towards the loading of the music library.
This new service was created for two reasons:
1. Music loading is slow and resource-intensive, so putting it on the
ViewModel layer just didn't seem right and made it vulnerable to the
OS simply stopping the loading process.
2. For automatic rescanning [#72], there must be something watching
the music library and waiting for a change in the background. This
would require a service as that is probably the least insane way to
do that kind of background work.
I have no garuntees how viable the service might be. If anything, it
might be halted by some insane android restriction or issue that
makes it more or less impossible to use for most apps, and I will have
to largely drop truly automatic rescanning.
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