[tali] snap: switch to core18

[Jeremy Bicha <jbicha src gnome org>]

commit 29eb767efaf7cb2003aeebb4ce55a6671eb6dda5
Author: Jeremy Bicha <jbicha ubuntu com>
Date:   Fri Feb 1 15:42:04 2019 -0500

    snap: switch to core18

 snap/snapcraft.yaml | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)
---
diff --git a/snap/snapcraft.yaml b/snap/snapcraft.yaml
index 54e9596..417cda0 100644
--- a/snap/snapcraft.yaml
+++ b/snap/snapcraft.yaml
@@ -11,12 +11,13 @@ description: |
 
 grade: stable # must be 'stable' to release into candidate/stable channels
 confinement: strict
+base: core18
 
 plugs:
-  gnome-3-26-1604:
+  gnome-3-28-1804:
     interface: content
     target: $SNAP/gnome-platform
-    default-provider: gnome-3-26-1604
+    default-provider: gnome-3-28-1804
   gtk-3-themes:
     interface: content
     target: $SNAP/data-dir/themes
@@ -45,8 +46,19 @@ apps:
     desktop: usr/share/applications/org.gnome.Tali.desktop
 
 parts:
+  desktop-gnome-platform:
+    source: https://github.com/ubuntu/snapcraft-desktop-helpers.git
+    source-subdir: gtk
+    plugin: make
+    make-parameters: ["FLAVOR=gtk3"]
+    build-packages:
+      - build-essential
+      - libgtk-3-dev
+    override-build: |
+      snapcraftctl build
+      mkdir -pv $SNAPCRAFT_PART_INSTALL/gnome-platform
+
   tali:
-    build-attributes: [no-system-libraries]
     after: [desktop-gnome-platform]
     source: .
     source-type: git