Platform preferences API

[Kevin Rushforth]

I'm finally catching up on the platform preferences API discussion, most 
of which is in Draft PR #1014 [1]. The information captured here [2] 
provides a good summary of the API.

A PR isn't really the best place to discuss the question of whether this 
feature is useful and whether the API is heading in the direction we 
want, so I wanted to have a high level discussion of that here.

First of all, I do see the value in having some way to provide the 
platform-specific properties like colors, etc, to the user. I'm less 
sure that the same class should also allow overriding those properties, 
but I can see the value of providing *some* way for the application to 
override them.

In order to move this forward, we need to answer the following questions:

1. Is this functionality something we want in the core of JavaFX? I 
think the answer is "yes", and it seems like others agree.

2. What is the best way to expose this capability? Currently it is 
proposed as an interface that extends ObservableMap (meaning that it 
*is* a Map rather than *contains* a map or has some other map-like 
representation). This seems fine to me, based on the discussion in the PR.

3. Should the platform preferences API provide the ability to set the 
properties (to override them) or should that capability be provided some 
other way (e.g., by some other class)? Perhaps this belongs as part of 
an eventual theming API, possibly via a "user preferences" class that 
extends or wraps the platform preferences class? At least the currently 
proposed API moved away from the odd "override" semantics and uses the 
standard "put" semantics. That, coupled with the two "reset" methods is 
a reasonable API, but I'm still not convinced that the core platform 
preferences class is the place for it. Please consider whether having a 
separate user / theme preferences interface / class, in addition to the 
(read-only) system preferences, might be better.

4. Is an Appearance enum the best way to provide the indication of LIGHT 
vs DARK? Do we need it as part of the Preferences API or should it wait 
until one of the later RFEs?

5. How do you anticipate this be used by an application prior to having 
CSS themes?

-- Kevin

[1] https://github.com/openjdk/jfx/pull/1014

[2] 
https://gist.github.com/mstr2/9f46f92c98d3c86aa6a0b4224a9a6548#platform-preferences