Notebook views UI (#13914)

* Add notebook editor

Introduce notebook editor component to allow for separate notebook displays in order to accomodate notebook views

* Localize notebook views configuration title

* Refactor view mode and remove the views configuration while it is unused

* Only fire view mode changed event when the value has been changed

* Remove notebook views contribution

* Add metadata capabilities

* Notebook views definitions

* Add notebook views models

* Views test

* Rename type arguments

* Additional tests

* Fix unused import

* Notebook views changes

* Add notebookviews.css

* Update notebookViewModel.ts

* Add cell toolbar styles

* Upgrade gridstack

* Add gridstack styles

* Remove ununsed references

These are part of the next PR and not available yet

* Remove gridstack static file

* Add gridstack as a module in the electron unit tests

* Spacing fixes

* Add copyright notice

* Remove commented code

* Spacing fixes in notebook styles

* Move handle svg to image

* Add typing for gridstack

* Move notebook styles to file

* Rename selector constant

* Rename grid css file

* Add nb-grid-stack class to views grid

* Cell toolbar style adjustments

* Remove unused imports

* Update .eslintrc.json

* Fix outdated instantiation of LabeledMenuItemActionItem

* Address feedback

* Fix from update to main

test/unit/electron/renderer.js

@@ -103,6 +103,7 @@ function initLoader(opts) {
 			'@angular/platform-browser-dynamic',
 			'@angular/router',
 			'angular2-grid',
+			'gridstack/dist/h5/gridstack-dd-native',
 			'ng2-charts',
 			'rxjs/add/observable/of',
 			'rxjs/add/observable/fromPromise',