* Added possibility to provide custom header back button's image, introduced API changes: removed old prop "headerBackImage", added new prop "backImage".
* Code style fixes.
* Renamed showcase file to for more clarity; updated the prop's name as well.
* Removed listeners code from showcase screens.
* wip
* fix imports
* use stylesheet for margin
stylesheet now correct
* also remove old headerLeft
* improve header buttons color
* remove ios button margin
* Add dismiss action, made possible by getParentState
* Add dismiss to flow interface
* Don't dispatch an action on dismiss helper if no parent state
* carefullyGetParent instead of getParentState
1. Remove `NavigationComponent` from `NavigationScreenRouteConfig`. The only context `NavigationScreenRouteConfig` is used in is as an intersection with an object, and as such the only relevant portions of `NavigationScreenRouteConfig` are the object parts.
2. Add static `HEIGHT` variable to `Header` type.
3. In `NavigationContainerProps`, make `onNavigationStateChange` property value nullable.
PS: if in the future you guys would prefer that I separate these sort of PRs into their constituent parts, let me know.
* Add activeLabelStyle and inactiveLabelStyle
Adding activeLabelStyle, so that active items can be customized more.
My use case: Change font of the active item to bold.
Also, added inactiveLabelStyle which can be used for a similar purpose.
* prettier fix
* Update react-navigation.js
* prettier fix
* Update jest snapshot for DrawerNavigator - for adding a new style property to the styles array
* This caches "child event subscribers" by key and removes them when they're not needed anymore. Once a navigator unmounts we also remove upstream subscribers.
* Fix tests
Should be nullable, since it is initially called as `null` in `src/createNavigationContainer.js` (and in `react-navigation-redux-helpers`, where it is causing a Flow error)
* use initialRouteName as key when initializing StackRouter
* fix null headerLeft
* merge back
* fixed tests
* use config flag
* fixed snapshots
* implemented requested changes
