Add a reference to the NuGet package WinRTXamlToolkit, which adds many controls missing in WinRT. Wire up the NumericUpDown control in this toolkit. However it instantiates, but still does not appear, possibly because its size of visibility need to be explicitly set.
Wire up the tree view section to WinRTXamlToolkit.TreeView. The section loads, but the tree is not currently visible.
Wire up the WinRTXamlToolkit's WebBrowser to WebBrowserHandler. As with the tree view, the section loads but the browser does not currently appear.
Xaml MessageBoxHandler, in progress - this needs to be async.

ApplicationHandler: cache dispatcher on creation.
Fix the tests page's content control to stretch horizontally and vertically, thus occupying the entire right pane.

Fix ButtonHandler to use a TextBlock for its text.