Support dnggainmap (embedded correction) for Bayer files (#6382)

* dng gainmap support, #6379
* dng GainMap: control sensitivity of checkbox, #6379
* dng GainMap: partial paste
* dng GainMap: moved isGainMapSupported() from dcraw.h to dcraw.cc
* RawImageSource::applyDngGainMap: small speedup
* Change GUI to separate gainmap from other flat-field; also reorder checkbox

Co-authored-by: Thanatomanic <6567747+Thanatomanic@users.noreply.github.com>

rtengine/stdimagesource.h

@@ -100,6 +100,11 @@ public:
         return false;
     }
 
+    bool        isGainMapSupported() const override
+    {
+        return false;
+    }
+
     void        setProgressListener (ProgressListener* pl) override
     {
         plistener = pl;