Build script for self-contained Mac .app (#185)

* Add CMake settings to build universal binary and find SDL in app bundle

* Source frameworks from extern folder

* Add build script, info plist and big sur style icon

* Add example build workflow (app won't be signed)

* Use macOS 11 runner for better compatibility

* Use better name for the build script, move metadata to Platform/macOS/ folder

* Improve macOS build instructions

* Fix workflow syntax and use a more specific glob pattern too

* Add libs search paths to CMakeLists.txt instead of find files

* Use same /Libs folder as libs search path

* Make build-mac-app.sh not fail on repeated runs

* Make build-mac-app.sh executable

.github/workflows/ReleaseBuilds.yml

@@ -0,0 +1,19 @@
+name: Make Release Builds
+
+on: [workflow_dispatch]
+
+jobs:
+  build-macos:
+    runs-on: macos-11
+    timeout-minutes: 15
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          ref: master
+
+      - run: bash build-mac-app.sh
+
+      - uses: actions/upload-artifact@v3
+        with:
+          name: mac-build
+          path: SpaceCadetPinball-*-mac.dmg