Mouuseenter, mouseleave for list and items are now always active and no 
longer added after the drag has started (they check of a drag operation 
is going on though). This makes the code simpler and more managable.

#634

#632

#631

#626

This fixes an issue where items could be added to only-one-item lists 
with replacement, when the  item to be replaced has the same list as 
origin.

#624

Fixed by using styles as in text field to define the background color

#625

#611

#611

#613

#611

#552

#621

#613

#618

- Prevent input of 'Dead' keys

- Remove unused imports

#599

Before it was just the bottom helper row.

#592

- New property group for flexible layout with more options to change the 
behaviour. It is now possible to build all other operations in this 
preset.

This is a free operation type that does not render much.

#590

#588

#588

Small cells and canBeCrossedOut are property of the row as they are all 
cells or none.
Additional property 'is2DigitHelperRow' is for different types of helper 
rows: single-digit in no crossing out or 2 digits with crossing out. 
2-digit is only for the top 2 substraction rows.

#589

- Minuend instead Subtrahend is strikable

#571

#578