Implement Session support through the extension host (#3228)

Full plumb through of Session support. Also fixed some test issues

- Load session and get necessary information in kernels list
- Run Cell button now works as expected
- Added a ToggleAction base class which can be used for anything that switches icons. I'd still prefer to have this be dynamic and as clean as the extension classes
- Fixed account test unhandled promise rejections (caused by incorrect / invalid tests) that made it hard to see all the test run output.

src/sql/parts/notebook/cellViews/code.css

@@ -25,6 +25,17 @@ code-component .toolbarIconRun {
 	background-image: url('../media/dark/execute_cell_inverse.svg');
 }
 
+code-component .toolbarIconStop {
+	height: 20px;
+	background-image: url('../media/light/stop_cell.svg');
+	padding-bottom: 10px;
+}
+
+.vs-dark code-component .toolbarIconStop,
+.hc-black code-component .toolbarIconStop {
+	background-image: url('../media/dark/stop_cell_inverse.svg');
+}
+
 /* overview ruler */
 code-component .monaco-editor .decorationsOverviewRuler {
 	visibility: hidden;