- allow 'wrong' input for unit edits where only one case makes sense (e.g. 10mHz for 10MHz)
- fix behavior when changing center of span would exceed the allowed range
- only change value for level or points when editing is finished (not on every keyboard input)
- Allow saving of calibration only if a calibration is active (no more
calibration files that "don't do anything" when they are opened)
- Delete old measurements when loading a new calibration file
- Update calibration when a measurement is updated (no need to disable
and enable again)
- Disable calibration when a required measurement is deleted
