Refactor MetaButtons

[ashmaroli]

Apart from simplifying the render() function, there are some other changes included in this proposal:

• MetaButtons now use an internal state to determine the dropdown visibility — the class no longer uses props onDropdownFocus and onDropdownBlur to toggle showing-dropdown className on the parent wrapper — the class is attached to the dropdown trigger button itself.
• The dropdown trigger toggles dropdown visibility — the dropdown gets hidden on both clicking the focused trigger and on the trigger losing focus.

[mertkahyaoglu]

Good one 👍