cola: remove notifier() from __init__

Signed-off-by: David Aguilar <davvid@gmail.com>
cola: remove single_selection() from __init__

Signed-off-by: David Aguilar <davvid@gmail.com>
cola: remove selection_model() from __init__

Signed-off-by: David Aguilar <davvid@gmail.com>
cola: remove selection() from __init__

Signed-off-by: David Aguilar <davvid@gmail.com>
cola: remove selected_group() from __init__

Signed-off-by: David Aguilar <davvid@gmail.com>
cola: move the cola.main package to cola.widgets.main and cola.models.main

Signed-off-by: David Aguilar <davvid@gmail.com>
cola: remove model() from __init__

Signed-off-by: David Aguilar <davvid@gmail.com>
browse: decorate the root

This brings back the drop-down indicator for the top-level folders.

Signed-off-by: David Aguilar <davvid@gmail.com>