Ability to style EditableTextBase's hint property

Right now there's either no clear way to style the placeholder hint text for EditableTextBase or that you can't. Particularly changing the colour. 

possible properties could be:
- `placeholder-color`

Unsure if these inherit from the normal font properties, but:
- `placeholder-font`
- `placeholder-font-size`
- `placeholder-font-weight`
- `placeholder-font-family`
- `placeholder-text-align`
