[Vala] Few bugs in Xnoise

[Tal Hadad <tal_hd hotmail com>]

After testing the new version, I have few bugs to report(sorry for being late):
1. Adding a new source directory to library should be async.
It means that after adding music directory, the user could continue and add another directory, not waiting 
for finishing scanning.

2. When (de/)activating the equalizer, the current track gets restarted.

3. I have not set a GTK3 thee in my machine, so it uses the default one.
Therefore, I see the ugly relief on the 4 media buttons, please disable this using "relief" property.

4. This is a suggestion, but think that the Ascending/Descending buttons are ugly and not interactive.
You should have an arrow down or arrow up in the button which is selected.
You can pack a Box(which contain a label(icon?) and arrow) inside a Button by using add () method.

Sorry for not being here long time.

Tal