Refactor and optimize CustomScriptPlugin::predefinedStyle[s]
Don't repeat the predefined style names in these two functions. Create the built-in styles once. Language plugins are still searched for styles each time, because they can be [un]loaded at run-time. This search should be fast because only one maintained plugin (kdev-python) creates a single source formatter style. Nothing prevents the language plugins from creating their styles once for optimization too.