DynamicResource support for app resource changes

[miloush]

Fixes #6302

Description

Changes in application resources are not picked up by DynamicResource extension if the target object is not in the visual tree (i.e. has no mentor). It already has code to access App/System resources in the absence of mentor which suggests this was meant to be a supported scenario.

This PR introduces internal ResourcesChanged instance event on the Application object and subscribes the ResourceReferenceExpression to the changes when the target element does not have a mentor.

Customer Impact

DynamicResource extension on DependencyObjects such as DataGridColumns or other user types will not track the changes to the application resources. However, the extension reads them correct the first time, making this issue difficult to diagnose.

Regression

No.

Testing

Built an updated PresentationFramework.dll and tested with the repro project in #6302 on 6.0.2 x86 app.
Built again and tested on 9.0.0 Preview 6 with changing system colors.

Risk

No changes to public API. No changes to behavior when the resource does not come from App/System resources, or when the target object has a mentor. At runtime, each instance of ResourceReferenceExpression will create a strong reference to the Application instance, but only in the affected case.

[batzen]

Doesn't the event subscription create a strong reference between application and expression, thus holding all expressions in memory for the lifetime of the application? Before the change the expression could be garbage collected as soon as the mentor could be. I guess this change will cause a massive memory leak.

[batzen]

Ah, i was blind...

[miloush]

How do we feel about this one, do we want this issue to be addressed and is this a reasonable way to do so? (It appears this is waiting on me for being a draft.)