fix dialog for selecting label icons up-scaling small icons

The chosen icon was converted to 48x48 even if the selected image was smaller.
Now resizing is only done if the image is bigger than 48x48
gstreamer: make the pulseaudio's volume monitoring optional as it is unstable

Couldn't find a fix for bug #54 that still happens in some cases (in particular when skipping though a song using the mouseheel),
so make the feature (introduced in commit #ffffd405e1 and partially fixed in #977c742e5d) optional and disabled by default
songtree: add $discname group variable

if discname field not enabled or no discname, it will try to make a discname using the disc number