network-manager: Fix all plugins to use gnome3 and gtk3, like nm-applet does

I think this makes sense, because now all the plugins will be gnome3 gtk3
based, the same way nm-applet is.

I also removed networkmanager_pptp_gnome variation of networkmanager_pptp
package, because i think no variation is needed and gnome support should
be on by default like in other packages.
Merge pull request #2708 from offlinehacker/pkgs/network-manager/gnome3

network-manager: Fix all plugins to use gnome3 and gtk3, like nm-applet does