Core UI enhancements and keyboard navigation

by Paul B for NetSurf

At the time being, all parts of the UI are created by the platform dependent code. This approach has some major drawbacks. NetSurf is designed for resource constrained hardware but it doesn't work with restricted input methods. At the moment a mouse is necessary for browsing. This can be a barrier for hand held devices and a serious inconvenience for advanced users. My intention is to remove the above limitations.