Add Full Screen feature

[wqafzal]

Title says it all. We need to show WebView Window in full screen but currently there is no such function provided.

The only solution for me would be to edit linux/webview_window.cc in .pub_cache which is not a good approach.
Thanks

[vukhacbiet]

+1

[cr0manty]

I created PR with Full Screen feature, but its only for Linux, if some one can take care about other platforms, I think that it will be merge soon

PR: #150