Ported treebrowser plugin to GTK+ >= 3.10.0

[paolostivanin]

now the treebrowser plugin uses GTK+ API compatible with version 3.10.0 and above. No more warnings during compilation :)

Tested on Arch x64, Ubuntu 14.04 x86, Ubuntu 15.04 amd64.

[b4n]

I don't like this. It adds a GTK 3.16 dependency for no good reason IMO.
Also, dropping all icons is questionable, but well, that's another topic.

[b4n]

At the very least the changes on the various plugins should be separate PRs.

[paolostivanin]

I didn't add GTK+ 3.16 functions :) I also think is not a good idea to use that version because *buntu 14.04 LTS users wouldn't be able to use Geany GTK+3 ;)

[paolostivanin]

Ok. So, can I close this pull request and open 3 new ones?

[b4n]

I didn't add GTK+ 3.16 functions :)

You use GtkLabel::xalign which was introduced by 3.16. Of course, you set it dynamically so it's not that bad, but it's kinda bad to set an non-existing property.

Ok. So, can I close this pull request and open 3 new ones?

Yes, but I myself won't merge them. As those are plugins not part of Geany core I won't oppose to them, but you'll need to convince someone else.

[paolostivanin]

I've already remove that code, it was my mistake :) I'll push it with the new prs I'm gonna create!
BTW, thanks for your time @b4n !