Add Gitlab CI and revise project demo structure
Demo version here: https://gitlab.com/ZingBallyhoo/KivyMD
Latest CI testing build: https://gitlab.com/ZingBallyhoo/KivyMD/commit/22d5b0b756f7050ae08322b3ef8916a4d413046f/builds
This PR adds automatic building of KitchenSink for android on Gitlab CI, to help with testing (meaning that we can quickly test things like MRs on android). Just push to any branch, and it will build the code at that commit into an APK. You can download the APK it generates by clicking the download button next to the build. Also moves KitchenSink into a new demos
folder, to make it easy to add more demos in the future (realistic examples?).