Feature: #79531 - EXT:form - Add multiselect inspector editor

See forge#79531


A new inspector editor, i.e. a new field type of the form editor, has been added. If applied, multi-select fields can be added to the inspector. A multi-select field allows the selection of multiple meta properties for a field and stores them in the defined property path.

For example: If you have a file upload element in your form, until now you could only select a single mime type restriction. With the new multi-select option, the mime-type field was converted and you can now select multiple mime-types (for example docx, doc and odt together).