runners/datetime: refine time zone matching

Initially submitted as part of !264 (merged) but it's actually a fix for some problems introduced with !292 (merged)

• match offset names only exactly, so 'time utc' wouldn't show 'UTC-10:00' (see !264 (comment 599426))
• show only city names (without continent) so relevance ranking works more accurately
• change delimiter for zones with country and comment from "/" to "-" for less ambiguity with separated regions
• use QStringLiteral QLatin1String instead of QString for composite names for efficienty