MythMusic: remove the local file access from findIcon()

This updates findIcon() to only look for the icons in the AlbumArt storage
group. Currently it only looks on the master I may change this to also look
on any slaves as well.

This also adds caching of the result of the search in a map to speed up the
lookups because searching for files over the myth protocol can be quite slow.
Support a 'homepage' in MythBrowser

Signed-off-by: Stuart Morgan <smorgan@mythtv.org>

Closes #8398
Fix the build after [bb8c246]. Botched hand merging of the patch.
MythMusic: update retrieving album art to use storage groups

This is just a quick fix to get it working again. It really needs reworking
to work better with storage groups.
AlbumArtImages: make sure the hostname is saved to the database
MythMusic: check we have found some tracks before showing some screens

Don't bother showing the playback or playlist editor screens if we haven't
found any tracks instead prompt the user to perform a scan.
Merge remote-tracking branch 'upstream/master'