Using the html property "disabled" results in a warning message in the
player when reactive forms are used at the same time.

Add new parameter 'itemsPerRow'. This limits the grid columns used, 
making items move to the next row if overextending.

Also simplify component structure.

A no longer existing class was used, but turned out to not be necessary 
anyway.

#313

Also add it to the likert row dialog.

#112

The dragged image did contain carry information about it's element.
Also remove useless drag and drop functionality for the placeholder.

- Remove prefix 'on'

This way elements can handle their logic themselves without having to
rely on outside utility classes.

Also restructure files in common in a more logical way.