Adding accessible context menus to TabView pages

[llongley]

In order for TabView pages to be sufficiently accessible, there needs to be a way for users to rearrange tabs using a single pointer input without the need for precise manipulation. To achieve this, I've added context menus for each TabView that allows users to move tabs left and right, as well as moving tabs to other windows in the case of the tab tear-out example.

In working on this change, I found that there's a bug in WinUI 3 where moving a TabViewItem from one TabView in one window to another in another window retains the previous XamlRoot value, which throws an exception when we try to open a context menu on that TabViewItem, since it doesn't know what its correct XamlRoot is. I've filed a bug on that issue, but for now, to work around that, I've switched from using explicitly defined TabViewItems to using a data item source, which causes each TabView to generate its own TabViewItems.

Finally, I also found a bug in Win32WindowHelper - newWndProc and oldWndProc should not be static, because the class itself is instantiated on a per-window basis. Having these be static can result in an EngineExecutionException as a result of these delegates being garbage collected while there's still a native reference to them. Changing them to be instance fields fixes this issue.

[niels9001]

@llongley FYI, looks like a conflict popped up