Add a design system (#34)

* Add theming system

* Add standard Button control and update RadioButtons

* Unify radiobutton with design system

* Update debug screen to have multiple views

* Add ToggleButton

* Update error controls to use design system

* Add typography to <Text> element

* Move DropdownButton into the design system

* Clean out old code

* Move Text into design system

* Add 'inverted' color palette

* Move LoadingPlaceholder into the design system

src/view/com/util/Selector.tsx

@@ -5,7 +5,7 @@ import {
   TouchableWithoutFeedback,
   View,
 } from 'react-native'
-import {Text} from './Text'
+import {Text} from './text/Text'
 import {colors} from '../../lib/styles'
 
 interface Layout {