Renderings settings 

This PR handles multiple rendering issues in Insight:
- The Gamma value which up to now could only be modified via the slider, can now also be entered manually in the corresponding text field.
- The Save button is now only enabled, if the rendering settings really have been changed. There is still one flaw: If the settings are changed and saved in the main viewer, the Save button in the metadata panel becomes enabled again; spent hours to figure out why that happens, without success, maybe I have an inspiration later.
- Simply renamed the save button to "Save" (was named "Apply" before).