StyleClasses reset accidentally by StylePolicies

**Describe the bug**
The style policies that are responsible for applying the style of components to morphs often override the `styleClasses` property, thereby erasing the effects of other properties that change the styleClasses in response to their change (i.e. `hideScrollbars`).

**To Reproduce**
Just initialize a component via `part()` that has its `hideScrollbars` property set to true. After calling `openInWorld()` which makes the style policy apply itself, the `styleClasses` property is reset to the default value.

**Setup**
- macOS
- Linux

**Version**: 54e8e6aff13c2dd56f95ea89ccb1d5b129766394