Give the preference dialog's filter field the full set of search-related
style bits (SWT.SEARCH | SWT.ICON_SEARCH | SWT.ICON_CANCEL) so it is
rendered as a native search field with a magnifier and cancel icon.

Also override setInitialText to expose the hint via Text#setMessage
only, instead of stuffing it into the field's content. The dialog
auto-focuses the filter field on open, so the old initial-text fallback
was never actually shown as a placeholder and forced users to delete
the hint before typing.