snapcraft.yaml: Add the GNOME 3.26 PPA.

Adds the PPA for the GNOME 3.26 framework which is maintained by the Ubuntu Desktop team.

This PPA includes GTK3 and GNOME3 components used to create the GNOME 3.26 Framework snap for Ubuntu 16.04.

  * Fixes:  microsoft#63194
  * Fixes:  microsoft#57019

linux/snap/snapcraft.yaml

@@ -10,7 +10,17 @@ grade: stable
 confinement: classic
 
 parts:
+  gnome:
+    plugin: nil
+    build-packages:
+      - software-properties-common
+    override-pull: |
+      add-apt-repository -y ppa:ubuntu-desktop/gnome-3-26
+      apt -y update
+
   code:
+    after:
+      - gnome
     plugin: dump
     source: .
     stage-packages: