Fix #2798 Load and Save extension issues.
- Removes dead-code from
extension/system.cpp
which could add extensions, but is never called to do so. - Move warning about opening files without an extension, so that it warns on failure, not on success
- Reverted bzr r10821 to fix issue with saving ALWAYS adding file extension even when checkbox for extension adding was deselected.
- Minor cleanup, translated code comment from German to English.
Fixes #2798 (closed)