Allow slicer visual style (i.e. drop-down) to be defined in JSON theme file

[morgan1195]

It would be nice if Power BI JSON theme files could control the visual style of slicers, such as setting them to 'Drop-down' by default.

Right now, it defaulst to 'List' style and changing them to 'Drop-down' has to be done manually for each slicer. This is inefficient, especially when building templates. Adding a simple property in the theme file like the following would allow for better automation, consistency, and cleaner UX: 
"slicer": {
    "style": "dropdown"
}

Please consider expanding the theme file capabilities to include visual-level configurations like this.