[WPF] Error when using nested TextEntry in Notebook sample in ScrollView.
Initial suppport for multi-toolkit image support

Image objects loaded using a toolkit can now be used in
other toolkits.
Add public method for creating multi-resolution images
Improve support for cross-toolkit images

Allow specifying an image loader callback. Secondary toolkits will use it to
load images on demand.
Fix NRE
Add ScrollToRow method to ListView/ListBox
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
Merge branch 'xtookit-images'
[WPF] Track api changes

Image.ToImageDescription() now takes an ApplicationContext as argument,
so we need to pass it along everywhere that method is used.
Merge pull request #339 from sevoku/add-list-scroll-to-row

Add ScrollToRow method to ListView/ListBox
[WPF] fix TextEntry PlaceholderText initialization
PlaceholderTextAdorner should be initialized after
the backend widget has bee fully loaded.
(fixes #329)
[MAC] add missing cursor position and selection support to TextEntryBackend
Add license file
Sensitivity for WindowFrame.
Example for Window.Sensitive
Merge pull request #350 from residuum/window-sensitivity

[WPF] [GTK] Sensitivity for WindowFrame
Merge pull request #343 from sevoku/fix-329

[WPF] Fixes #329