Add indent and outdent buttons to editor toolbar

As shown on https://gitlab.gnome.org/Teams/Design/app-mockups/blob/master/notes/notes-aday.png

It's enabled for everything but maybe it should just be enabled for lists to create nested list

I'm not sure about putting the commands in biji_toggle_block_format() since it's not really a toggle, but the logic in the function is similar.

I was also wondering if the definition of the XML namespace should be updated but I couldn't find where it's stored.

Fixes #22 (closed)