Select UX review changes

Some of the changes that we discussed while reviewing the new Select component.

Styles

Select element has not-allowed cursor even though not disabled, should have default cursor
Center search dropdown (currently alligned left)
Selected option in single-select looks very dim, should be black
Tag styles/padding (@nguyensomniac to update the designs)
Active single select should maybe have border/highlight (@nguyensomniac to update designs)
Select & highlighted items should have different styles (@nguyensomniac to update designs)
Make sure disabled state works as per https://github.com/uber-web/baseui/pull/198

Keyboard

Single-select should have tabindex so it can be focused (multi-select is okay since it has input)
When TYPE.select is focused, down arrow and spacebar should open dropdown
Backspace should delete an item in multi-select
Pressing enter on highlighted item should select/deselect it.
Escape should close the dropdown for both single and multi-select

General Functionality (some reported by users) (Added by @abmai)

Dropdown begins in an open state
For TYPE.search dropdown does not re-open after closing
Selecting an object should return entire object not just id and label
For TYPE.search selecting an option does not clear current input value
Single-mode TYPE.search not supported? We should have something similar to default behavior of react-select Single mode

Stories

Search doesn’t seem to work reliably in the first multi-select storybook example. Maybe we should do a fixed set of options in first story, and have a separate story for demonstrating async results?
Images and labels are overflowing tag in custom label story
Clear the search term on item selected or dropdown closed, make the closing configurable
Add a search/single mode example