Provide database index for widgets which is used by widget queries to
fetch data way faster, e.g. 0.1 instead of > 5 seconds in one concrete
project.
Relates: #63
As it most likely is the most used configuration.
Therefore it should be as easy as possible.
Also it matches more or less any Extbase plugin with nested structure.
Allow integrator to limit results in widget to certain languages.
If only one language is allowed, labels will be translated.
Otherwise default system language is used.
E.g. one record will be displayed in multiple languages, it would be
confusing to show him multiple times for each language.
Add TYPO3 extension for phpstan as many things would break within tests.
Relates: #15
