Rebuild the vaadin-contextmenu event with native APIs

[tomivirkki]

Needs vaadin/vaadin-context-menu#219

The current implementation uses Polymer's Gestures. In order to drop that dependency, rebuild the event with platform native API's

[Haprog]

The blocking issue linked in the description seems to be gone and the link doesn't work. I wonder which issue that was?

[tomivirkki]

This repository now includes the gestures module, so the dependency is no longer there. Closing as no longer relevant.