extend WebView functionality (incl. all backends)
* load custom html content
* go back/forward, reload, stop loading
* new properties: Title (incl. changed event), LoadProgress, CanGoBack, CanGoForward
* additional navigation and loading events
* 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
[GTK] fix RemoveSignalHandler extension method
[WPF] implement WebViewBackend.Title using script invoke
[MAC] fix url handling for OnNavigateToUrl
[Xwt] Fixed/Added Tree Operations:
- Xwt.GtkBackend.TreeStoreBackend: Added GetPrevious implementation
- TreeNavigator - Added MoveToFirstSibling/MoveToLastSibling.
- TreeStore - Added InsertNodeAfter/InsertNodeBefore.
[Xwt][GtkBackend] Fixed GetPrevious using TreePath ( lluis suggestion https://github.com/mono/xwt/pull/355#discussion-diff-14774581 )
Merge pull request #355 from mono/fix_tree_operations

[Xwt] Fixed/Added Tree Operations:
Merge pull request #341 from sevoku/extend-webview

extend WebView functionality (incl. all backends)