Fixed issues around GTK Stock being deprecated

I tried building this on Ubuntu 14.04 and ran into compilation issues around the use of the GTK_STOCK parts which were depricated in gtk 3. Just a simple replacement means this now builds and now works great.

*edit*: I also stuck it on launchpad, and that seems to build the deb without issue: https://code.launchpad.net/~growse/networkmanager-ssh/master