Message Content Rework

For now everything should look identical. However this moves to using a model for the content of the message and is intended to lay the foundation for improved message content representation, e.g. splitting up a text message in multiple sections and using different delegates for things like code and quotes.

src/qml/MimeComponent.qml

@@ -7,6 +7,9 @@ import QtQuick.Layouts
 
 import org.kde.kirigami as Kirigami
 
+/**
+ * @brief A component to show media based upon its mime type.
+ */
 RowLayout {
     property alias mimeIconSource: icon.source
     property alias label: nameLabel.text