[FEATURE] Settings for default variant and version

This change introduces three new master defaults under `plugin.tx_fluidcontentcore.settings.defaults`:

* `mode` which can be `record`, `preselect` or empty. If `record` the following values get saved to the DB when the record is saved with empty selections. If `preselect` the selector fields will be preset with the chosen default selected. If empty, the defaults are applied on-the-fly if record's values are empty, and are not saved to DB.
* `variant` which can be set to an extension key which provides the default variants you want to use.
* `version` which can be set to the desired default version of content elements inside the chosen variant.

Needless to say, entering an invalid value here - for example an extension key which isn't loaded - causes the infamous "INVALID VALUE: (foobar)" message in selector fields.