META: Handle theme-specific configuration

Problem/Motivation

Given the interoperability aims of Features 3.x, and given that interoperable features should not assume a particular theme, it's not immediately clear how theme-specific configuration should be handled.

Beyond interoperability, the same basic issues apply any time a features-based site has a default theme or admin theme distinct from those in the feature(s): if we include configuration for a specific theme, sites with other themes won't receive upstream updates.

Theme-specific configuration includes:

• Block configuration entities.
• Theme-specific simple configuration.

Proposed resolution

Possible approaches (some of which are mutually exclusive):

• Package admin-theme related configuration in a particular feature, perhaps 'site'.
• In the Exclude plugin, remove all theme-specific configuration for themes that are not either the default theme or the admin theme.
• Designate a given core theme as the theme of reference, and export theme-specific configuration for that theme. For example, if Bartik is the core theme of reference, an article-related block configuration for Bartik might go in an 'article' package. Sites running such features with a different theme would then need to synchronize their blocks with those of the designated core theme. See Block Theme Sync.
• Add a UI feature for each non-UI feature, with any configuration for the default theme going in the UI feature.

Remaining tasks

User interface changes

API changes