Improvements to user-settings for mapcss mappaint styles

[Klumbumbus]

User-settings for mapcss mappaint styles were included in JOSM with very basic functionality (see #10421). While this was ok for its first use, it could need some improvements which I want to sum up here:

• Currently it is only accessible in mappaint dialog. Many users do not use the dialog or even do not know how to get it. (Although I described where to change the user setting and attached a screenshot ​in the forum, users searched in the preferences window to change the user setting and gave up searching). It would be useful if the user-settings are also accessible in the preferences window and in the view menu (maybe also for buttons placed in the toolbar). To make it accessible in the preferences window (which is maybe indeed the first place to search for this setting) is maybe relevant for milestone:14.08 since it includes the big changes in default mappaint style.

• Extend the user settings to numbers, lists, colors and strings. Styles/Lane_and_Road_Attributes would need the colors.

• Enable to set a keyboard shortcut to change a specific (boolean) user setting.

• Make translations with tr() also working for external mappaint styles which are managed in this wiki Styles#StylesmanagedinJOSMwiki. (I'm not sure if this is already working, I didn't try, but I guess it doesn't work yet.)