Speedup for opening music song library

Previously, this repeatedly parsed the same URL string over and over again
for every song, each time the list of songs was opened.

Benchmarks for CMusicDatabase::GetSongsByWhere() on a Raspberry Pi with a
library of 4115 songs:

Before patch: 14.5 seconds
After patch:   4.0 seconds
Merge pull request #3391 from jmarshallnz/faster_getsongs

Speedup for opening music song library