BlazorWebView WPF tab navigation improvements

[MackinnonBuck]

Description of Change

Prior to this PR, the WPF BlazorWebView was not correctly handling tab navigation. The BlazorWebView control itself was participating in tab navigation in accordance with its IsTabStop and TabIndex properties, but its subtree (the underlying WebView2 control and its focusable HTML elements) was always last in the tab sequence and unaffected by those properties.

This PR changes the WPF BlazorWebView tab navigation behavior in the following ways:

• By default, the BlazorWebView control itself cannot receive focus, but its children can. Prior to this change, using <tab> to move the focus from a control outside the WebView to the first HTML element inside the WebView required two presses, since the first press moved focus to the BlazorWebView container and the second press moved it to the first child.
• Previously, setting IsTabStop="False" on the BlazorWebView would remove the container from the tab sequence, but not its children. Now, doing this removes all the BlazorWebView's children from the normal tab sequence. If you manually give an HTML element focus (e.g. by clicking it), you can navigate the webpage using the keyboard, but as soon as keyboard navigation escapes the BlazorWebView, it will stay that way (unless focus is somehow given to an HTML element again).

Issues Fixed

Fixes #2537

[MackinnonBuck]

For the sake of reviewability, I've recorded a small demo showing the use of tab/shift+tab to navigate a simple WPF app with a BlazorWebView:

tab_navigation.mp4

[Eilon]

I don't know how you figured this out, but this is amazing!

[javiercn]

Looks great!

Great job figuring this out!