message: Use gnome-platform content snap for depending GTK
  Adopt the gnome-platform content snap can decrease the snap disk usage
  by sharing the same GNOME/Gtk libraries with other snaps.  After the
  modification the snap size shrinks from ~132MiB to ~111MiB.
  The recipe of the gnome-platform snap:
diff: @@ -61,6 +61,11 @@ plugs:
  gsettings:
  mount-observe:
  + gnome-3-26-1604:
  + interface: content
  + target: $SNAP/gnome-platform
  + default-provider: gnome-3-26-1604:gnome-3-26-1604
  +
  parts:
  poedit:
  after:
  @@ -69,7 +74,7 @@ parts:
  - gtkspell
  - patches
  - wxwidgets
  - - desktop-gtk3
  + - desktop-gnome-platform
  source: .
  @@ -131,6 +136,10 @@ parts:
  --file "${SNAPCRAFT_STAGE}"/patches/postprocess-desktop-entries.sed \
  "${SNAPCRAFT_PART_INSTALL}"/share/applications/net.poedit*.desktop
  + build-attributes:
  + # Gtk libraries provided by gnome-platform content shared snap
  + - no-system-libraries
  +
  stage-packages:
  # Called by Poedit
  - gettext
  @@ -254,6 +263,9 @@ parts:
  - libtiff5-dev
  - libxtst-dev
  - zlib1g-dev
  + build-attributes:
  + # Gtk libraries provided by gnome-platform content shared snap
  + - no-system-libraries
  plugin: autotools
  override-build: |
  @@ -465,6 +477,10 @@ parts:
  prime:
  - $localization
  + build-attributes:
  + # Gtk libraries provided by gnome-platform content shared snap
  + - no-system-libraries
  +
  # Self-built Enchant Part #
  # This part provides a patched recent version of the Enchant spell-
  # checking library required by the GtkSpell library. It workarounds
  