map new CSS properties:
- `color` on `IColorElement` (`ActivityIndicator`, `BoxView`),
`ProgressBar` and `Switch`
- `row-gap` and `column-gap` on `Grid`
- `transform` and `transform-origin` on `VisualElement`
- `vertical-align` on `Label`
add new XF-specific CSS properties
- `-xf-placeholder` and `-xf-placeholder-color` on `Editor`, `Entry`,
`SearchBar`
- `-xf-max-length` on `InputView` (`Editor` and `Entry`)
- `-xf-bar-background-color` and `-xf-bar-text-color` on
`NavigationPage` and `TabbedPage`
- `-xf-orientation` on `ScrollView` and `StackLayout`
- `-xf-horizontal-scroll-bar-visibility` and
`-xf-vertical-scroll-bar-visibility` on `ScrollView`
- `-xf-min-track-color`, `-xf-max-track-color` and `("-xf-thumb-color`
on `Slider`
- `-xf-spacing` on `StackLayout`
This PR replaces and closes#3276
- fixes#2891