Add CI task definition to build an AppImage again

Putting the main logic into a second Makefile should make it easier to move away from Github CI later.

All of this is untested so far, although it is migrated from the old .travis.yml, so the approach should work.

.github/workflows/appimage.yml

@@ -0,0 +1,16 @@
+name: Create Appimage
+on: [push]
+jobs:
+  Create-Appimage:
+    runs-on: ubuntu-20.04
+    steps:
+	  - name: Install dependencies
+	    run: sudo apt-get -y install pkg-config libglib2.0-dev libgconf2-dev libgtk-3-dev libwnck-3-dev libwxgtk3.0-gtk3-dev libxcb1-dev libxcb-ewmh-dev xcb-proto librsvg2-dev
+	  - name: Compile software
+	    run: make
+	  - name: Install into appdir
+	    run: DESTDIR=appdir make install
+	  - name: Build AppImage
+	    run: make -f Makefile.appimage build
+	  - name: Upload AppImage
+	    run: make -f Makefile.appimage upload

Makefile.appimage

@@ -0,0 +1,13 @@
+.PHONY: build upload
+
+build:
+	@wget -c -nv "https://github.com/probonopd/linuxdeployqt/releases/download/continuous/linuxdeployqt-continuous-x86_64.AppImage"
+	@chmod a+x linuxdeployqt-continuous-x86_64.AppImage
+	@unset QTDIR; unset QT_PLUGIN_PATH ; unset LD_LIBRARY_PATH
+	@export VERSION=$(git rev-parse --short HEAD)
+	@./linuxdeployqt-continuous-x86_64.AppImage appdir/usr/share/applications/*.desktop -appimage
+
+upload:
+	@find appdir -executable -type f -exec ldd {} \; | grep " => /usr" | cut -d " " -f 2-3 | sort | uniq
+	@wget -c https://github.com/probonopd/uploadtool/raw/master/upload.sh
+	@bash upload.sh Simdock*.AppImage*