Issue #2659940 by almaudoh, alexpott, phenaproxima, Jo Fitzgerald, markcarver, oriol_e9g, dawehner, dsnopek, jibran, larowlan: Extension System, Part III: ThemeExtensionList and ThemeEngineExtensionList

Issue #2208429 by dawehner, almaudoh, alexpott, jibran, Xano, donquixote, amitgoyal, bircher, stefan.r, Mile23, alvar0hurtad0, Berdir, andypost, larowlan: Extension System, Part III: ExtensionList, ModuleExtensionList and ProfileExtensionList