refactor: change the toolkit directive prefixes and remove Md prefix from toolkit classes (#2258)

* refactor: change the toolkit directive prefixes and remove Md prefix from toolkit classes

Renames the the component toolkit to use the `cdk-` prefix, in addition to removing the `Md` prefix from classes associated with the toolkit.
The following CSS classes have been renamed as well:

* `md-global-overlay-wrapper` -> `cdk-global-overlay-wrapper`
* `md-overlay-container` -> `cdk-overlay-container`
* `md-overlay-pane` -> `cdk-overlay-pane`
* `md-overlay-backdrop-showing` -> `cdk-overlay-backdrop-showing`
* `md-overlay-dark-backdrop` -> `cdk-overlay-dark-backdrop`
* `md-overlay-backdrop` -> `cdk-overlay-backdrop`
* `md-visually-hidden` -> `cdk-visually-hidden`

* Re-export Md prefixed classes as deprecated

* Fix linter errors.

* Change the overlay variable prefixes to `cdk-` and remove dependency on theming.

* Add backwards compatible selectors for core directives

e2e/components/dialog/dialog.e2e.ts

@@ -91,7 +91,7 @@ describe('dialog', () => {
     browser.actions()
       // We need to move the cursor to the top left so
       // the dialog doesn't receive the click accidentally.
-      .mouseMove(element(by.css('.md-overlay-backdrop')).getWebElement(), { x: 0, y: 0 })
+      .mouseMove(element(by.css('.cdk-overlay-backdrop')).getWebElement(), { x: 0, y: 0 })
       .click()
       .perform();
   }

e2e/components/menu/menu-page.ts

@@ -14,7 +14,7 @@ export class MenuPage {
 
   triggerTwo() { return element(by.id('trigger-two')); }
 
-  backdrop() { return element(by.css('.md-overlay-backdrop')); }
+  backdrop() { return element(by.css('.cdk-overlay-backdrop')); }
 
   items(index: number) { return element.all(by.css('[md-menu-item]')).get(index); }