Update theme of Add Widget dialog when system theme changes

[krschau]

Summary of the pull request

If the user updated the system theme while the Add Widget dialog was open, the dialog would not update. This change fixes the problem by:

• Update the dialog itself. Before we were setting the RequestedTheme to be the ActualTheme, which meant if the RequestedTheme was Default, it would not update when the Default changed.
• Update the widget preview image by listening to ActualThemeChanged and swapping out the image.
• Update the widget icons by listening to ActualThemeChanged and swapping out the Content of the menu items.

This change has the nice side-effect of decoupling the AddWidgetView and the DashboardView, by requiring the DashboardView to pass in the dispatcherQueue or ActualTheme anymore. It will also make it easier to move more logic from the AddWidget View to ViewModel in the future if we want to.

References and relevant issues

Detailed description of the pull request / Additional comments

Validation steps performed

PR checklist

• Closes Add Widget dialog doesn't respond to theme changes while open #2406
• Tests added/passed
• Documentation updated

[AmelBawa-msft]

Do we need to try catch widget definition usage, or we don't expect RPC errors here?

[krschau]

Technically yes, I think we should, but I think the only way to do this without making the whole dashboard a mess of try/catches is to wrap the widget objects in a model so we don't have to be querying the real objects the entire time (which I do plan to do at some point).