Add support for QTextBlockFormat::marker

This patch enables use of QTextBlockFormat::marker introduced in Qt 5.14.
Using it, any list element can be marked with interactive checkbox.

It adds a KToggleAction with which user can toggle this checkbox.

Brief explanation of the logic:

• If the cursor is already on a list, we just toggle the marker
• If the cursor is not on a list, we have to add a list first (having a marker without a list just does nothing)
• If the cursor was on the single-element list, and we have disabled the marker, disable the list too.