Avoid self:: calls to be able to extend the sources form ContentEntitySourcePluginUi class

Thanks for the patch, now it is possible to extend the class when overriding UI class for Content Source plugin, for example when trying to add more filters, like "Published/Unpublished", etc.

Reviewed and thanks for provided patch. That will make extending and overriding the existing functionality more easier. I hope it wil be merged in the nearest time.

Patch in #2 tested and working! :)

Committed.

Note that extending this plugin might break in the future as there are issues where we're adding our own additional filters.

Automatically closed - issue fixed for 2 weeks with no activity.