Add toast message UI.

[na9da]

What this PR does

Part of UX for #6845
Split from #7626

• Adds a new toast message UI.
• Adds new traits to show initialMessage as a toast instead of a popup.

{
 "catalog": [
   {
     "id": "bike-racks-toast",
     "type": "geojson",
     "name": "GeoJSON Test",
     "url": "test/bike_racks.geojson",
     "initialMessage": {
       "title": "Bike racks",
       "content": "This message will self-destruct in 5 seconds",
       "showAsToast": true,
       "toastVisibleDuration": 5
     }
   }
 ]
}

Test me

• Open this CI link

• Add the only item from the catalog

• Observe the notification toast that auto-dismisses
  

• Remove and add the item again to the workbench

• Observe that the toast is not shown again (only the first time)

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.

[micholm]

Tested with initialMessage requiring acceptance and works as expected. LGTM

http://ci.terria.io/toast-notification/#clean&https://gist.githubusercontent.com/micholm/461f12b41d16c926a458f90e390fc571/raw/e5009cad424f98d016cf499e5ba6393ca4f56c19/toasttest.json