This stuff was all over the place, a relic from before the editor
settings were unified. Now the initialization is only done in
TextEditorSettings.

Fixes the .pro file editor and the generic project files editor, which
before only used the font settings.

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

forward declaration is sufficient.

Details:  - Naming convention

Details:  - Conform to naming convention

Details:  - To make it conform to naming convention.