extend WebView functionality (incl. all backends)

This PR contains following new WebView features:
* load custom html content
* go back/forward, reload, stop loading
* new properties: Title (incl. changed event), LoadProgress, CanGoBack, CanGoForward
* additional navigation and loading events (incl load interception support)
* add all new functionality to samle apps
* fix Gtk.Windows backend if inside scrolled container

Backend limitations/TODOs:
WPF: custom html doesn't work (wpf bug?)
WPF: Title needs mshtml binding
GTK: WebKit.WebView on resize: content grows but does not shrink
MAC: not tested at all