[RFC] Add a datepicker interface to JOSM

[taylor.smock]

See comment:36:ticket:14921 for a potential usecase in core.

I've got an implementation over here: ​https://gitlab.com/smocktaylor/josm_datepicker/-/blob/master/src/main/java/org/openstreetmap/josm/plugins/datepicker/IDatePicker.java .

Would it be a good idea to put that in core? If so, I'll make some modifications and post it.

Notes: Currently uses java.time.LocalDate instead of java.util.Date. There probably won't be a UI, at least initially, to set the config value that determines the appropriate class to initialize.

Right now, my intent is to use this in the Mapillary plugin to remove the JavaFX dependency. I will probably post a patch for the JavaFX plugin that will enable it to be used as a date picker without being a hard dependency for other plugins.

Alternatively, I can just use it as a plugin, but I'd like to avoid having Mapillary depend upon libraries that are not modifiable by core developers.

So I would like to put the plugin in a location where core developers can make modifications to it if I cannot (i.e., a bus hits me).

Options:

1) OSM SVN. Not an option (#19509)
2) Add the plugin to the JOSM GitHub group, probably with a bidirectional mirror on Gitlab's end (I dislike using Github)
3) Wait for the JOSM Gitlab instance to be spun up and become available for plugin authors (#16857). This would make Mapillary depend upon out-of-tree plugins for some period of time.