feat(modal): focus is now trapped inside any opened modal

Closes #642
Closes #2441

demo/src/app/components/modal/demos/basic/modal-basic.html

@@ -1,15 +1,27 @@
 <ng-template #content let-c="close" let-d="dismiss">
   <div class="modal-header">
-    <h4 class="modal-title">Modal title</h4>
+    <h4 class="modal-title">Profile update</h4>
     <button type="button" class="close" aria-label="Close" (click)="d('Cross click')">
       <span aria-hidden="true">&times;</span>
     </button>
   </div>
   <div class="modal-body">
-    <p>One fine body&hellip;</p>
+    <form>
+      <div class="form-group">
+        <label for="dateOfBirth">Date of birth</label>
+        <div class="input-group">
+          <input id="dateOfBirth" class="form-control" placeholder="yyyy-mm-dd" name="dp" ngbDatepicker #dp="ngbDatepicker">
+          <div class="input-group-append">
+            <button class="btn btn-outline-secondary" (click)="dp.toggle()" type="button">
+              <img src="img/calendar-icon.svg" style="width: 1.2rem; height: 1rem; cursor: pointer;"/>
+            </button>
+          </div>
+        </div>
+      </div>
+    </form>
   </div>
   <div class="modal-footer">
-    <button type="button" class="btn btn-outline-dark" (click)="c('Close click')">Close</button>
+    <button type="button" class="btn btn-outline-dark" (click)="c('Save click')">Save</button>
   </div>
 </ng-template>
 

src/modal/modal-window.ts

@@ -13,6 +13,7 @@ import {
 } from '@angular/core';
 
 import {ModalDismissReasons} from './modal-dismiss-reasons';
+import {ngbFocusTrap} from '../util/focus-trap';
 
 @Component({
   selector: 'ngb-modal-window',
@@ -44,6 +45,7 @@ export class NgbModalWindow implements OnInit,
 
   constructor(@Inject(DOCUMENT) document, private _elRef: ElementRef<HTMLElement>, private _renderer: Renderer2) {
     this._document = document;
+    ngbFocusTrap(this._elRef.nativeElement, this.dismissEvent);
   }
 
   backdropClick($event): void {