[UX] Language switcher block: allow some basic customization via the block configuration form

This was brought up as a forum question: https://forum.backdropcms.org/forum/language-switcher-how-change-looks-l...

My new Backdrop website will be bilingual (Dutch/English). The default language switch block is huge and doesn't look nice:

It is not so obvious to me how to tweak the appearance. I guess I have to use the configuration dialogue box for the Language switcher

...

How do I get a simple NL / EN switch like this one:

I guess the answer will be that I need to add some CSS style definitions, but how?

I would like to propose adding a couple of checkboxes to the block dialog, to allow for the following customizations:

• displaying language links as bullet points, or as a delimited list (using a pipe |)
• rendering the full name of the languages, or the respective language codes