Use Java 8 compatibility build for SNAP build due to Ubuntu 16.04 limitations (no JDK 11, no JFX 11, etc)

installer/snap/snapcraft.yaml

@@ -8,31 +8,45 @@ grade: stable
 
 architectures: [amd64]
 
-apps:
-  filebot:
-    command: desktop-launch $SNAP/bin/filebot.sh
-    plugs: [desktop, desktop-legacy, wayland, unity7, x11, gsettings, dbus, home, removable-media, network, network-bind]
-
 plugs:
-  gtk-3-themes:
+  gtk-2-engines:
+    interface: content
+    target: $SNAP/lib/gtk-2.0
+    default-provider: gtk2-common-themes:gtk-2-engines
+  gtk-2-themes:
     interface: content
     target: $SNAP/share/themes
-    default-provider: gtk-common-themes:gtk-3-themes
+    default-provider: gtk-common-themes:gtk-2-themes
   icon-themes:
     interface: content
     target: $SNAP/share/icons
     default-provider: gtk-common-themes:icon-themes
-  sound-themes:
-    interface: content
-    target: $SNAP/share/sounds
-    default-provider: gtk-common-themes:sounds-themes
 
 environment:
+  GTK_PATH: $SNAP/lib/gtk-2.0
+  GTK_DATA_PREFIX: $SNAP
   XDG_DATA_DIRS: $SNAP/share:$XDG_DATA_DIRS
 
+apps:
+  filebot:
+    command: desktop-launch $SNAP/bin/filebot.sh
+    plugs: [desktop, desktop-legacy, x11, gsettings, dbus, home, removable-media, network, network-bind]
+
 parts:
   filebot:
     plugin: dump
     source: base
-    stage-packages: [openjdk-11-jre, openjfx, libmediainfo0v5, libchromaprint-tools, p7zip-full, p7zip-rar, xdg-utils, curl]
-    after: [desktop-gtk3]
+    stage-packages: [openjdk-8-jre, openjfx, libmediainfo0v5, libchromaprint-tools, p7zip-full, p7zip-rar, xdg-utils, curl]
+    after: [desktop-gtk2]
+  extra:
+    plugin: dump
+    override-build: |
+      set -ex
+      snapcraftctl build
+      mkdir -p $SNAPCRAFT_PART_INSTALL/lib/gtk-2.0
+      mkdir -p $SNAPCRAFT_PART_INSTALL/share/themes
+      mkdir -p $SNAPCRAFT_PART_INSTALL/share/icons
+    stage:
+      - lib/gtk-2.0
+      - share/themes
+      - share/icons