More flexible Headline marks#196

Make marks optional, we don’t want clients to mess up carefull crafted headline typography with unwanted formatting, do we?

Would be user friendly to be able to select the Headline level from within the marks. This saves time and I’m sure clients will only use the default Headline level from now on and mess up structure (if there is any).

Marks can be set to false by extending the default headline block:

https://forum.getkirby.com/t/disable-toolbar-in-block-type-heading/20549/3

Writer has headings option to limit the available headline levels