Rename compilePerFile to incremental and update its default in gwt:run-*

gwt:compile and gwt:test still default to non-incremental compilation,
like in GWT proper.

Document jsInteropMode as being an experimental flag.
Add incremental and jsInteropMode to gwt:run
Add support for modulePathPrefix in gwt:run

Automatically suffix webappDirectory with the provided modulePathPrefix
so gwt:compile outputs the modules to the same directory (provided
webappDirectory and hostedWebapp have the same value).