lxterminal: switch to using GTK3

The version of VTE that uses GTK2 is not maintained, so using lxterminal
with GTK2 means dealing with a lot of bugs that have already been fixed
in newer VTE versions. I actually meant to set up lxterminal to use GTK3
in the first place, but didn't realize that wasn't the default until
now.

pkgs/applications/misc/lxterminal/default.nix

···
-{ stdenv, fetchurl, automake, autoconf, intltool, pkgconfig, gtk2, vte
+{ stdenv, fetchurl, automake, autoconf, intltool, pkgconfig, gtk3, vte
 , libxslt, docbook_xml_dtd_412, docbook_xml_xslt, libxml2, findXMLCatalogs
 }:
 
···
 
   configureFlags = [
     "--enable-man"
+    "--enable-gtk3"
   ];
 
   nativeBuildInputs = [
···
     libxslt docbook_xml_dtd_412 docbook_xml_xslt libxml2 findXMLCatalogs
   ];
 
-  buildInputs = [ gtk2 vte ];
+  buildInputs = [ gtk3 vte ];
 
   patches = [
     ./respect-xml-catalog-files-var.patch

pkgs/top-level/all-packages.nix

···
   };
 
   lxterminal = callPackage ../applications/misc/lxterminal {
-    vte = gnome2.vte;
+    vte = gnome3.vte;
   };
 
   deepin-terminal = callPackage ../applications/misc/deepin-terminal {