Make BlaVisualization a singleton class

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Refactor visualization code

This commit refactors the handling of visualizations. It gets rid of the
plug-in-like handling of visualization modules. From now, on we'll only
support spectrum visualizations.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
blaplaylist.py: change playlist status layout

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
blabrowsers.py: remove "Properties" option

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
blafm.py: avoid three unhandled exceptions

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
blareleasebrowser.py: small PEP8 fix

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
blaguiutils.py: rewrite BlaTreeViewBase (again...)

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Integrate the tag editor into the sidebar

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Remove call to deprecated BlaPlaylist.add_tracks

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Rename BlaTreeViewBase's `allow_no_selection' arg

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Add "Clear queue" button to the statusbar

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Rename BlaLibrary's `update_library' method

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>