Allow extensions to contribute to the color themes

Currently extensions cannot add new entries to the color theme.
Consequently if an extension needs some appearance settings, only two choices are available:

• reuse existing color theme values (not very flexible)
• define cross-theme configurations (and possibly not consistent with most of the color themes, this is what's done by the Mentions and Syntax Highlighting extensions).

Some UIXP should be defined to allow extension to contribute to the color themes.