CAConfig Editor: Add "dropdown list" widget

currently every data type usable in caconfig editor has a fixed, simple edit widget (despite the pathbrowser widget).

we want to add a new widget type dropdown which allows to define a list of options with value/description pairs in the parameter definition. the user can pick one of the option using a dropdown list showing the descriptions, internally the value is stored.

the dropdown widget type should be usable with string or number data types.

for multi-valued types (array types) the dropdown list should allow multi-selection. (if this is difficult in the UI we can skip this requirement and do not support array types for this feature.)

if an existing configuration contains a value that is not part of the value/description list from the definition a new entry should be added automatically to the list with the stored value as value and description to make it visible that a value not present in the list is currently stored.