fix #584 show albums and tracks when filtering for genres via search text

[dermeck]

This is split into two commits because I'm not sure if the second commit is "too much" and should be reverted.

the first one is the actual fix:

• adds genres field to album model
• considers genres field of track/album when the respective filter-pipe is used

the second one is a rafactoring:

• first collects the relevant text and only calls the searchService once, which (subjectively) also makes the test a bit easier

[digimezzo]

Thanks for this PR! And sorry for the delay in reviewing this. I'll review it this week.

[dermeck]

No worries - I'm aware that this is a side project :) .

[digimezzo]

@dermeck Thanks again for this PR. The fix looks great! I'm merging it.