Myth Protocol: add optional allowFallback parameter to QUERY_SG_FILEQUERY

A value of '0' will prevent it falling back to searching every storage group
if not found in the specified one. If omitted or a value of '1' is passed then
it will fall back to search all storage groups.

format is now:
QUERY_SG_FILEQUERY <host> <storagegroup> <filename> <allowfallback (optional)>
RemoteFile::FindFile: make sure we only search the storage group we want
PlaylistEditorView: don't use itemVisible if using the tree view

This will make sure we don't hunt for node images and icons if we are using the
tree view which doesn't show them.

It would be nice to only disable the hunt if the buttonimage image is not
present in the buttonlist but I can't see a way to do that.
UPNP: Fix music streaming following the switch to storage groups
RemoteFile: Prevent FindFile causing the master backend connecting back to itself and therefore failing to find the file.
UPNP: Fix upnp albumart after move to storage groups.
MythMusic: fix filename for embedded images after a file scan
mythutil: make sure we only use the AlbumArt storage group in ExtractImage
MythMusic: add a check for the AlbumArt storage group

Don't show any of the music screens before a valid AlbumArt directory is
defined. We already check for the Music storage group so this just extends
that.
Playlist: silence a failed query warning when inserting an empty playlist

Seems QSqlQuery treats QStrings that haven't been assigned to as NULL not an
empty string so 'QString rawList;' is not the same as 'QString rawList = "";'
as far as QSqlQuery is concerned.
MythMusic: add a 'Reset Database' button on the general settings page

This will clear the music tracks database tables so you can start afresh.
It doesn't clear the smart playlists or radio stream tables.
mythutil: send a MUSIC_ALBUMART_CHANGED message in ExtractImage

This changes the message on successfully extracting an image from a tracks tag
and saving it to the cache to MUSIC_ALBUMART_CHANGED. Sending a
MUSIC_METADATA_CHANGED message can result in an infinite loop under some
circumstances.
mythutil: Silence a compiler warning in CalcTrackLength()
EditMetadata: handle MUSIC_ALBUMART_CHANGED messages in the albumart editor
Merge remote-tracking branch 'upstream/master'

Conflicts:
	mythplugins/mythmusic/mythmusic/generalsettings.cpp
	mythplugins/mythmusic/mythmusic/generalsettings.h
	mythplugins/mythmusic/theme/default-wide/musicsettings-ui.xml
	mythplugins/mythmusic/theme/default/musicsettings-ui.xml