Instead of simply wiping all control themes and styles that are applied to a control, we can now just remove the `ValueFrame`s which relate to the control theme that was changed.
To do this, added `ValueFrame.FramePriority` which encodes both the `BindingPriority` and source of the frame (style, control theme, templated parent control theme).
- Removes the `IStyler` service and the `Styler` implementation
- Moves the logic for applying styles and control themes into `StyledElement`
- Removes the style `TryAttach` method from the public API
- Removes style caching for now - this will need to be added back
Now tries to simulate an application with a lot of styles applied at different points in the logical tree.
Make `StyledElement.ApplyStyling` a public API in order to do this.
- Don't use Rx in the styling system. Instead introduces `IStyleActivator` which is like an `IObservable<bool>`-lite in order to cut down on allocations.
- #nullable enable on touched files
Max Katz
Steven Kirk
Nikita Tsukanov
Dariusz Komosinski
Steven Kirk
Benedikt Schroeder