BUG #15059: Bring back lost comments about ${TIME}.

Added placeholder text to Chart Title widget to indicate using ${TIME}
for time. Also added the same to the tooltip.

This added a new xml hint "PlaceholderText" to add placeholder text for
pqLineEdit in pqStringVectorPropertyWidget.

Change-Id: I20cae97ecb07c80ed7c44872fea39a876d60bcf9
Merge topic '15059_fix_time_comments' into next

dc7a344d BUG #15059: Bring back lost comments about ${TIME}.