Currently when dropping elements onto a part stack MStackElements are removed if their element ids are equal.
This leads to disappearing parts in case of
- at least one part created using part descriptors allowing multiple instances is present in the part stack
- another such part is dropped onto the part stack

Following the comments/notes the intention is to remove placeholder parts with the same element id as the
element's id being dropped.

This fix ensures that only instances of type MPlaceholder are removed from the part stack.

Fixes #2771