Allow default DisplayModeFallbacks to be changed through code

We wanted to use EPiBootstrapArea but change the default DisplayModeFallback modes that the tool already registers.

I'm not sure whether your Registrar covered this scernario, it seemed to only cover adding additional DisplayModeFallbacks.  I wanted to start from nothing and define my own in code.

I've added in a IDisplayModeFallbackProvider which is responsible for providing the default values to register. I also had to change the way the registration works so it cleared all entries from the DDS. (I also kind of wonder why the DDS is needed, and you couldn't persist in memory?)

As a user you would have to provide your own concrete IDisplayModeFallbackProvider and initialise this as per this gist https://gist.github.com/markeverard/7201ec4b5c80d35b614c

Never used Xunit before so didn't provide any tests :(