Applying styling in the constructor isn't a good idea as demonstrated by #8549..
Instead apply styling when showing a window, or if it's needed call `ApplyStyling` manually, e.g. in unit tests.
Fixes#8549
- 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
Fixes for #8092:
- Always round sizes up, not to the nearest pixel, thereby ensuring that `DesiredSize`s don't get rounded down where possible.
- Apply rounding to `Padding` and `BorderThickness` in measure pass as well as arrange pass, to ensure that `DesiredSize` takes this rounding into account.
When a native popup window is positioned relative to a placement target on
the root window and the root window is moved or resized, the popup position
should update to reflect the new position of the placement target.
If a native popup window is positioned relative to another popup, as in the
case of sub-menus, and the parent popup is moved due to a change in the
position of its placement target, the child popup should also update
relative to the new position of the parent popup.
A few `AutoScrollToSelectedItem` improvements:
- Scroll to current selected item when it's set to true
- Scroll to current selected item when list first displayed
- Scroll to current selected item when attached to visual tree if the selection was changed while it wasn't attached
Fixes#4100