[applets/digitalclock] Custom Time Format Support

This adds support for custom time formats to the digital clock and reworks how seconds and 24/12 hour time is setup.

1. Backwards Compatability: Is it okay that we removed the showSeconds option and reset the user's time settings?

2. Visual Design:

   

   • Does it look fine?
   • Are the hh/mm/ss specifiers able to be understood by the general user?
   • For reference, Windows looks like:

3. Intervals: Previously the interval was set by the showSeconds config option: interval: plasmoid.configuration.showSeconds ? 1000 : 60000. Now, this is not possible. I see three options:

   • Always update every second (doesn't work with the possible of the millisecond specifier)
   • Allow configurable intervals (works for all but a bit unwieldy)
   • Detect whether a specifier is used and autoconfigure based on that. See: Qt's AP recogniser
   • This needs to be removed as a TODO from the code before merging.

4. Time Format Documentation: Would it be beneficial to include the format specifiers in a popup rather than linking to Qt's documentation? Especially given that the documentation link doesn't work; it doesn't scroll to the right part of the page. (an issue with qt's docs atm it seems)

5. Time Displayed: I just chose an arbitrary time in the afternoon. This keeps it consistent as otherwise switching between will have the value change.

I think I had another point to discuss but forgot about it.