Adds `.screen()` plugin API method for adding plugin-provided
screens. Such screens only have common headers and footers, while rest
of the page is filled with the registered web component.
Any navigation to URL containing `#/x/` hash results in client-only
redirect to `/x/` and showing appropriate plugin-provided screen.
Adds a `plugin.screenUrl()` method for generating consistent URLs for
such screens.
Adds an example plugin with number of ways the provided API can be used.
Adds partial support for GWT UI `.screen()` method. Notable difference -
does not support RegExp for the screen matching.
Change-Id: I0be3dee8eba6f8535a1fb2be05f473f3649bad8f
6ea8c8a31e