Advanced preferences: display default entries consistently

[bastiK]

In the advanced preference dialog, we do not show keys and default values for all settings that exist in the software. Instead we fill the list at runtime, whenever the value of a setting is requested in the code.

I noticed this is irritating and users often don't understand this concept.

It would be better, if the list of advanced preferences would consistently show all (or nearly all) available entries.

Proposed solutions:

1. Central registry: Basically convert all Main.pref.get(...) calls to StringProperty and put those properties in a class that is loaded early.
2. When JOSM exits, write all known defaults to a cache file (~/.josm/cache/default_preferences.xml) and load these entries back at next start.