Add custom heading style to player and editor

This is rather hard to implement via HTML inline style attributes. So we 
do it the classic way.
Could be reviewed, when that feature is implemented in TipTap:
https://github.com/ueberdosis/tiptap/issues/1514.

projects/editor/src/styles.css

@@ -14,4 +14,19 @@ body {
 .drop-list {cursor: grab}
 .drop-list.cdk-drop-list-dragging {cursor: grabbing}
 
-.mat-menu-panel {border: 2px solid red}
+h1 {
+  font-weight: bold;
+  font-size: 20px;
+}
+h2 {
+  font-weight: bold;
+  font-size: 18px;
+}
+h3 {
+  font-weight: bold;
+  font-size: 16px;
+}
+h4 {
+  font-weight: normal;
+  font-size: 16px;
+}

projects/player/src/styles.css

@@ -2,3 +2,20 @@ body {
   overflow: hidden;
   margin: 0;
 }
+
+h1 {
+  font-weight: bold;
+  font-size: 20px;
+}
+h2 {
+  font-weight: bold;
+  font-size: 18px;
+}
+h3 {
+  font-weight: bold;
+  font-size: 16px;
+}
+h4 {
+  font-weight: normal;
+  font-size: 16px;
+}