Yet another set of misc changes and fixes

New features

• HTML template corresponding to each PolymerTemplate is parsed to verify that there's actually an element corresponding to any @Id annotation value.
• Integrated initial version of a code generator that will eventually be used for generating Java APIs for all Core Elements
• Imported contribution guidelines from Vaadin Framework 8 and adjusted to suit Flow
• BootstrapListener now has access to VaadinUriResolver.

Fixes

• Fixed a case where @Id injected elements were not fully functional in all cases
• Fixed path resolution on Windows for legacy AngularTemplate cases.
• PolymerTemplate no longer inherits a broken implementation of HasChildView.setChildView(View).

Breaking changes

• PolymerTemplate no longer directly inherits HasChildView.
• Some constants have been moved between com.vaadin.server.Constants and com.vaadin.shared. ApplicationConstants.

Demos

• Added Google signin demo

All changes

https://github.com/vaadin/flow/milestone/34?closed=1