Fix for possible duplicated IDs in the search input boxes

Using the idPrefix to guard against duplicate IDs if two similar grids
are added to the same page.

(if using same column name(s))

Works well, even when no idPrefix is specified (defaults to nothing)

Similar to issue: https://github.com/tonytomov/jqGrid/issues/397, however that issue propose a major re-factoring - I only modified 5 Places to make it html compliant.