Reorganize the settings model in the VCS base plugin, store model in the plugin for better delayed initialization/update.

Fixes: Add common configuration to the VCSA base plugin; submit message check script and user name configuration.                                           Details: Extend submit editor widget by configureable fields. Use them in the VCS base submit editor to specify users, provide completion and selection dialog for them.

replace ExtensionSystem::PluginManager::instance()->getObject<Core::ICore>() by Core::ICore::instance()