Specialized buttons add-on for Vaadin 14

This addon implements some specialized buttons:

• HoldButton - a Button that needs to be pressed for a specified time.
• SlideButton - a Button simulating "swipe to unlock" style.
• DelayedButton - a Button that becomes active only after a delay. Shows a countdown in caption.
• CancellableButton - a Button that you can cancel during given period. Shows a countdown in caption.

Development instructions

1. Import to your favourite IDE
2. Run the Maven goal mvn jetty:run http://localhost:8099
3. Code and test

• create UI's for various use cases for your add-ons, see examples. These can also work as usage examples for your add-on users.
• create browser level and integration tests under src/test/java/
• Browser level tests are executed manually from IDE (JUnit case) or with Maven profile "it" (mvn verify -Pit).

4. Test also in real world projects, e.g. create a demo project, build a snapshot release ("mvn install") and use the snapshot build in it.

Creating releases

1. Push your changes to e.g. Github
2. Update pom.xml to contain proper SCM coordinates (first time only)
3. Use Maven release plugin (mvn versions:set -DnewVersion=4.0.3; mvn install -Pdirectory)
4. Upload the ZIP file generated to target/checkout/target directory to https://vaadin.com/directory service (and/or optionally publish your add-on to Maven central)