Improve "Plasmid features" dialog in sequence view

It appeared, that "Plasmid features" have a lot of problems:

• The dialog is terrible - no information about how sequences we are about to searching for look like. It is just several checkboxes, nothing more.
• The documentation is terrible either - it has an external link, which is broken and leads us to some unrelated page. And doc has an information that we can check "data/custom_annotations/plasmid_features.txt" if we want to see plasmid features, which is as bad as possible, because users should not even know anything about UGENE inner kitchen.
• As far as I can understand, it was anticipated, that user should change the above-mentioned file to add its own plasmid features (there is "Over features" checkbox, which is not related to any feature from the file), but it is almost impossible on macOS, because this file is read-only.

This dialog (and, of course, documentations) should be impoved. User should see features they want to search and have possibility to add their own. Documentations should describe everything very good.