editor

    Details:  before that it was the plugin, so there was a lot of
inactive handlers left around

    Details:  the installHandler() overload was missing an implementation

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

It was getting a bit cluttered without any grouping of the different
actions. Also, the collapse and expand actions have been added, mainly
for discoverability.

forward declaration is sufficient.

writing of partial files for now.

Details:  - Naming convention

Details:  - Conform to naming convention

Details:  - To make it conform to naming convention.