[GTK3] use cairo context provided by Gtk instead of creating a new one for canvas drawing
[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)
[GTK] cleanup GtkWebKitMini.cs
[GTK] update libwebkitgtk reference and dllmaps
to enable plain Gtk.WebView support on windows and mac
(thx@ lytico)
[GTK] stop using deprecated libwebkitgtk api
(thx@ lytico)
[Samples] fix WebView url filter example
(relative urls produced an exception)
Extend color selector example
[GTK] add color selector backend
fix HSL conversion in DefaultColorSelectorBackend
extend DefaultColorSelectorBackend
* replace text entries with spin buttons
* add opacity slider
add more examples to samples app
add spin button examples
Merge branch 'gtk3'
Merge branch 'add-color-selector-backends'

Conflicts:
	Xwt.Gtk/Xwt.Gtk.csproj
Merge branch 'extend-webview'

Conflicts:
	Xwt.Gtk/Xwt.GtkBackend/GtkInterop.cs
Merge branch 'extend-samples'

Conflicts:
	TestApps/Samples/Samples/ButtonSample.cs
	TestApps/Samples/Samples/Checkboxes.cs