Refactorings to prep for a later show_seconds PR

[FriendOfEntropy]

Related to #208

Per discussion in a previously canceled show_seconds PR, this is a set of small refactorings to limit the amount of simultaneous changes for review when a new show_seconds PR is created later.

• Parse_time will become especially complex and busy when second support is added, so making it a bit more succinct here.
• Going ahead and adding a with_format creation version of TimePicker to the demo view to help with regression testing, demonstrate the usage in code. When show_seconds is added later, there will be a with_seconds equivalent of this being demonstrated

[donadigo]

Can you explain why the timepicker_with_format was added to the demo? It displays the same thing for me as the time_format. Is this something for the refactor?

[jeremypw]

@danrabbit This is related to an issue you raised, so I requested a review from you. Not sure if the author is still available to work on it.

[jeremypw]

I am going to close this as the author has shown little or no activity since beginning of 2020 and no review has been forthcoming.