Change default style of React select

[bierett]

In the past, we have updated the React Select dropdown button which is used in the basemap switcher. The change affects only the basemap switcher package (f2b4169).

Old style

New style

We want to use the change for all packages, which used the React Select dropdown button (for all exisiting and future implementations).

Idea: Add pointer cursor and white dropdown button to the base theme for all React Select.

[mbeckem]

Would require a small wrapper component to enforce common styles and css class names etc.