Improve a11y

**Issue Type:**

Enhancement

**Description:**

We should improve a11y across all components. This includes components like buttons, dropdowns and search inputs. We should add a subtle indication of focus/active actions such as adding a slight box-shadow based on the theme color. We can also refer to other UI libraries such as bootstrap or react-semantic-ui for inspiration.

**Screenshots:**



**Minimal reproduction of the problem with instructions:**

**Reactivesearch version:** x.y.z

**Browser:** [all | Chrome XX | Firefox XX | Edge XX | IE XX | Safari XX | Mobile Chrome XX | Android X.X Web Browser | iOS XX Safari | iOS XX UIWebView | iOS XX WKWebView ]

**Anything else:**