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

.gitignore

@@ -266,7 +266,7 @@ __pycache__/
 /Export
 /DrMem
 /Doc private
-# Windows local libraries
+# Windows and macOS local libraries
 /Libs
 
 #CMake generated