Issue #2278541 by tim.plunkett, kim.pepper, Bojhan, EclipseGc: Refactor block visibility to use condition plugins.

Issue #2256679 by alexpott, tim.plunkett: Fixed Use config schema to determine which config entity properties to export regardless of whether they are public or protected.

Issue #2283929 by tim.plunkett: Clean up condition plugins to have schema support and to allow them to be optional.

Issue #2274175 by tim.plunkett, vijaycs85: Block plugin schema should be defined separately from the entity.

Issue #2264179 by Gábor Hojtsy, vijaycs85, alexpott, benjy: Clarify use of property/undefined and add an ignore type in configuration schema.

Issue #2236089 by cs_shadow, sriharsha.uppuluri: theme_settings schema duplicates the label Shortcut icon settings where it doesn't make sense.

Issue #1067408 by alexpott, Jalandhar, jessebeach, Désiré, neetu morwani, dawehner, sun: Themes do not have an installation status.

Issue #2233787 by alexpott, xjm | tim.plunkett: Move default configuration into extension/config/install to clarify its purpose.

Issue #2218655 by sun, effulgentsia, jessebeach: Core expects $theme.settings default config file to exist.

Issue #2228921 by sun: Consolidate system.module + system.theme + system.theme.disabled into core.extension config.