feat(dialog): align with 2018 material design spec (#12705)

Aligns the dialog with the most-recent Material Design spec.

BREAKING CHANGES:
* The `matDialogAnimations.slideDialog` symbol has been renamed to `matDialogAnimations.dialogContainer`.

![angular_material_-_google_chrome_2018-08-16_20-43-44](https://user-images.githubusercontent.com/4450522/44231568-da0d2780-a19e-11e8-9836-4f1b15f9bcc2.png)
![angular_material_-_google_chrome_2018-08-16_21-30-17](https://user-images.githubusercontent.com/4450522/44231576-e09b9f00-a19e-11e8-9fae-8ad9cf0be8e5.png)

Author: crisbeto

src/cdk/overlay/_overlay.scss

@@ -6,7 +6,7 @@ $cdk-z-index-overlay: 1000;
 $cdk-z-index-overlay-backdrop: 1000;
 
 // Background color for all of the backdrops
-$cdk-overlay-dark-backdrop-background: rgba(0, 0, 0, 0.288);
+$cdk-overlay-dark-backdrop-background: rgba(0, 0, 0, 0.32);
 
 // Default backdrop animation is based on the Material Design swift-ease-out.
 $backdrop-animation-duration: 400ms !default;

src/demo-app/dialog/dialog-demo.scss

@@ -2,3 +2,12 @@
   max-width: 405px;
   margin: 20px 0;
 }
+
+button {
+  margin-right: 8px;
+
+  [dir='rtl'] & {
+    margin-left: 8px;
+    margin-right: 0;
+  }
+}

src/lib/dialog/dialog.scss

@@ -3,7 +3,7 @@
 
 
 $mat-dialog-padding: 24px !default;
-$mat-dialog-border-radius: 2px !default;
+$mat-dialog-border-radius: 4px !default;
 $mat-dialog-max-height: 65vh !default;
 $mat-dialog-button-margin: 8px !default;
 
@@ -44,9 +44,11 @@ $mat-dialog-button-margin: 8px !default;
 }
 
 .mat-dialog-actions {
-  padding: $mat-dialog-padding / 2 0;
+  padding: 8px 0;
   display: flex;
   flex-wrap: wrap;
+  min-height: 52px;
+  align-items: center;
 
   // Pull the actions down to avoid their padding stacking with the dialog's padding.
   margin-bottom: -$mat-dialog-padding;