runners/datetime: rework display of time zones

- Also find time zones by short name and offset name
- Display country results by region name (country name and comment if applicable), and for countries having multiple time zones (e.g. Mexico), show all of them
- Display abbreviation results together their long name to disambiguate (e.g. "Pacific Standard Time (PST)" instead of "PST"), and for abbreviations having multiple time zones (e.g. PST), show all of them (BUG: 462836)
- For date matches, show difference to local date
- For time matches, display date difference as "+/- 1 day" if applicable and none otherwise, instead of always displaying full date
- Sort results by relevance based on match length, so that e.g. with query "gmt+1", "GMT+14" wouldn't be higher up than "GMT+1"
- Find and display time zone names with " " instead of "_"
- Display long lines wrapped rather than elided
- Make time difference strings fully localizabe
- Some minor reformatting, refactoring and code clean-up