Support execution count in run button and align correctly (#3979)

Fixes #3931 
- Align run button correctly so it's centered in new cell
- Refactor to support multi-state button.
  - Hidden state is set to show execution count
  - Stopped state shows run button
  - Running state shows stop button
  - Error state (will) show error button. This isn't fully handled right now
- Add execution count to model and to SqlKernel, verify serialization, loading, update matches other notebook viewers

**Notes on implementation**:
I think this is a decent solution for a) showing execution count as text, and b) perfectly centering the run button.
 
The below solution shows count correctly up to 999 runs (that’s clicking 999 times in a single session), the icon lines up juuust about right with [ ] but for other numbers it is pretty close but probably not exactly right. I wish I could solve this to work better but trying to change text float to center etc. really isn’t working.
 
**Screenshots**:
![image](https://user-images.githubusercontent.com/10819925/52466366-e8794200-2b36-11e9-9a50-86893e75d5af.png)

With running cell:
![image](https://user-images.githubusercontent.com/10819925/52466378-f333d700-2b36-11e9-9e6c-3cee098790fd.png)

src/sql/workbench/api/node/mainThreadNotebookDocumentsAndEditors.ts

@@ -542,7 +542,7 @@ export class MainThreadNotebookDocumentsAndEditors extends Disposable implements
 					uri: cell.cellUri,
 					contents: {
 						cell_type: cell.cellType,
-						execution_count: undefined,
+						execution_count: cell.executionCount,
 						metadata: {
 							language: cell.language
 						},