Fixed KT-5611 Completion doesn't include not imported extensions for implicit receiver

 #KT-5611 Fixed
KT-5876 Code completion should never auto-insert item which requires adding import

 #KT-5876 Fixed
Code completion: avoid duplication of elements for java classes
Class renames
One more class rename
Completion: the same universal insertion handler is used for java classes too
Fixed bug in reference shortening + fixed test data for completion tests
Removed method which is not needed anymore
Refactored completion tests to not set/restore auto-complete settings in 2 different places
Removed some more stuff from ImportInsertHelper
Minor
Checked that KT-3596 is not reproducible