Qt core18 workaround

https://forum.snapcraft.io/t/desktop-qt5-can-not-be-used-for-remote-part-based-on-core18

snap/snapcraft.yaml

 name: solarus
-version: 1.6
+version: '1.6.2'
 summary: A free and open-source Action-RPG game engine
 description: |
   Solarus is a free and open-source Action-RPG game engine, licensed under GPL,
   and written in C++. It runs quests in Lua, and can work on a great number
   of platforms. Solarus also features a game editor called Solarus Quest Editor,
   written in C++ with Qt to help you create your game.
+base: core18
 
 confinement: strict
 
@@ -55,10 +56,6 @@ parts:
       echo "deb http://ppa.launchpad.net/litenstein/sdl2-xenial/ubuntu xenial main" | tee /etc/apt/sources.list.d/litenstein-ubuntu-sdl2-xenial-xenial.list
       apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 1E0A4B32AA748B9D11C4528301A509285F2EA337
 
-      # Qt upgrade (5.12.3)
-      echo "deb http://ppa.launchpad.net/beineri/opt-qt-5.12.3-xenial/ubuntu xenial main" | tee /etc/apt/sources.list.d/beineri-ubuntu-opt-qt-5_11_1-xenial-bionic.list
-      apt-key adv --keyserver keyserver.ubuntu.com --recv-keys C65D51784EDC19A871DBDBB710C56D0DE9977759
-
       # Install updated packages
       apt-get update
       apt-get install -y libsdl2-2.0-0 qt512-meta-full
@@ -109,3 +106,29 @@ parts:
       sed --in-place 's|^Icon=.*|Icon=${SNAP}/share/icons/hicolor/256x256/apps/solarus-quest-editor.png|' share/applications/solarus-quest-editor.desktop
       # Move `assets` directory
       mv share/solarus-quest-editor/assets bin/assets
+  desktop-qt5:
+    build-packages:
+    - build-essential
+    - qtbase5-dev
+    - dpkg-dev
+    make-parameters:
+    - FLAVOR=qt5
+    plugin: make
+    source: https://github.com/ubuntu/snapcraft-desktop-helpers.git
+    source-subdir: qt
+    stage-packages:
+    - libxkbcommon0
+    - ttf-ubuntu-font-family
+    - dmz-cursor-theme
+    - light-themes
+    - adwaita-icon-theme
+    - gnome-themes-standard
+    - shared-mime-info
+    - libqt5gui5
+    - libgdk-pixbuf2.0-0
+    - libqt5svg5
+    - try:
+      - appmenu-qt5
+    - locales-all
+    - xdg-user-dirs
+    - fcitx-frontend-qt5