Support for dynamic placeholders on jqms

Problem/Motivation

Currently there is no support for dynamic placeholders if multiple select boxes are being used and we need separate placeholders for each select box.

Steps to reproduce

Add some multiselctbox to page.
Navigate to jqms configuration page where placeholder text can be added.
Notice that there is only one option to add placeholder text and it will be reflected on all selectbox.

Proposed resolution

This attached patch will fix the above issue by supporting separate placeholders for different multi select boxes.

Remaining tasks

User interface changes

API changes

Data model changes