albumArtCache: Don't resolve the thumbnail key

Workaround for the random segfault when resolving the thumbnail key.
Populate methods in grilo.py resolves this key too, so this part could be
safely removed.
Revert "When updaiting album art update it for all items"

This reverts commit 2fb580bec64c3c1d3eb90a93db91b85fa3f9338c.

This commit loads the covers in Albums view multiple times. The workaround for
resolve will now fix the bug fixed by this commit.
player: Mark "Unknown Artist" as translatable

Also, changed it from "Unknown artist" to "Unknown Artist" to match other
instances of this in other source files.