Reduce calls to LLHoverView::updateText

Basically to avoid calling LLTrans::getString 250 times per second,
and seeing
INFO: log_sStringTemplates_accesses: LLTrans::getString/findString called 19000 in total.
INFO: log_sStringTemplates_accesses: LLTrans::getString/findString called 20000 in total.
etc, every 40 seconds in the log.