Add preference to allow going past 100% volume

[cadadr]

This is something PulseAudio allows, but before this patch, you needed to open the mixer to make use of this feature.

This commit adds a preference which toggles increasing volume beyond 100%, which when turned on, modifies the Gtk.Scale widgets to go up to 150, with a marker indicating where 100% is.

When enabled, this is how it looks like:

As the todo comment in the patch indicates, IIRC it's possible to even go beyond 150% volume, so it might make sense to make that number configurable also, but that'd rarely be used I suppose.

[illfygli]

Nice.