WIP: merged templated perspective feature back into uberfire-workbench-client

It is now possible to mix and match templated and classic perspectives in the same app.

There are still some correctness problems that I'm woring out, and a lot of tests are still to come.
But the basic refactoring has proven a success.
Fixed splash screen lifecycle and perspective switching problems with templated root panel
Modified uberfire-client-webapp to show how to use templates with LayoutPanels

I'm still not happy with the situation. It's too awkward that you have to supply
your own panel/dev with relative layout. The framework's panels should do this
themselves, somehow.