Add button to show task addition pane.

src/app/main-page/main-page.component.html

@@ -1,7 +1,12 @@
-<div>
+<div class="component">
     <h1>Todo List</h1>
     <div *ngFor="let task of tasks" [title]="task.creationDate">
         <h1>{{ task.title }}</h1>
         <p>{{ task.description }}</p>
     </div>
+    <div id="add-task-btn" *ngIf="!isAddingATask" (click)="startTaskAddition()">
+        <mat-icon>add</mat-icon>
+        <span>Ajouter une nouvelle tâche...</span>
+    </div>
+    <app-add-task *ngIf="isAddingATask"></app-add-task>
 </div>