extensions specify default toolbar button context

extensions/ohif-cornerstone-extension/src/ToolbarModule.js

@@ -90,5 +90,6 @@ const definitions = [
 ];
 
 export default {
-  definitions
+  definitions,
+  defaultContext: 'ACTIVE_VIEWPORT::CORNERSTONE'
 };

extensions/ohif-vtk-extension/src/toolbarModule.js

@@ -37,5 +37,6 @@ const definitions = [
 ];
 
 export default {
-  definitions
+  definitions,
+  defaultContext: 'ACTIVE_VIEWPORT::VTK'
 };