Fix #2798 Load and Save extension issues.

1. Removes dead-code from extension/system.cpp which could add extensions, but is never called to do so.
2. Move warning about opening files without an extension, so that it warns on failure, not on success
3. Reverted bzr r10821 to fix issue with saving ALWAYS adding file extension even when checkbox for extension adding was deselected.
4. Minor cleanup, translated code comment from German to English.

Fixes #2798 (closed)