Use gwt appcache to provide Offline support

This pull requests demonstrates replacing the existing MGWT Appcache support with that provided by the gwt-appcache library (https://github.com/realityforge/gwt-appcache). 

It is NOT ready yet to be merged but is intended to give you a feel for what such a change would look like if it were to be merged. I took this strategy as I had emailed you 2-3 times and had yet to receive a response so I thought this may be a more useful way to start the conversation ;) 

Essentially I extracted the appcache support from mgwt and refactored it significantly to meet our demands. Essentially I needed functionality to run offline but not necessarily mobile (offline desktops and apps built into vehicles feature heavily in what I want to do). Now I would like to see what it would take to get it integrated back.

To get this pull request into a merge-worthy state, there would need to be;
* more extensive testing across a broader range of platforms (so far my focus has been on desktop chrome/firefox/safari)
* migration documentation for existing MGWT users

I am willing to do this work if it is a path you feel this approach is worth pursuing.

Thoughts?