fix(limel-dialog): fix broken markup and resulting styling of the header

The slot>header>h1-h6 structure is not supported by MDCDialog. Change to h2 only to support correct
styling by MDC.

BREAKING CHANGE: While backwards compatible in most cases, some functionality has been removed. If
the `heading`-attribute is not set, the text-content of anything injected into the header-slot is
copied and placed in the new h2-element. But any use of `<em>`, `<strong>`, or similar, will be
stripped. So will any `<a>` tags. But none of these should be used in a dialog-heading anyway.

src/components/dialog/dialog.scss

@@ -2,6 +2,10 @@
 @import "@lime-material/typography/mdc-typography";
 @import "@lime-material/dialog/mdc-dialog";
 
+slot[name="header"] {
+    display: none;
+}
+
 .mdc-dialog__surface {
     max-height: 95vh;
     width: 70rem;

src/components/dialog/dialog.tsx

@@ -14,6 +14,12 @@ import {
     styleUrl: 'dialog.scss',
 })
 export class Dialog {
+    /**
+     * The heading for the dialog, if any.
+     */
+    @Prop({ reflectToAttr: true })
+    public heading: string;
+
     /**
      * `true` if the dialog is open, `false` otherwise.
      * Defaults to `false`.
@@ -71,12 +77,7 @@ export class Dialog {
             >
                 <div class="mdc-dialog__container">
                     <div class="mdc-dialog__surface">
-                        <header
-                            class="mdc-dialog__title"
-                            id={'limel-dialog-title-' + this.id}
-                        >
-                            <slot name="header" />
-                        </header>
+                        {this.renderHeading()}
                         <div
                             class="mdc-dialog__content"
                             id={'limel-dialog-content-' + this.id}
@@ -105,6 +106,13 @@ export class Dialog {
             this.mdcDialog.close();
         }
     }
+
+    private renderHeading() {
+        if (this.heading) {
+            return <h2 class="mdc-dialog__title">{this.heading.trim()}</h2>;
+        }
+        return null;
+    }
 }
 
 function createRandomString() {