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.