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/rawimage.h

@@ -245,11 +245,6 @@ public:
         return zero_is_bad == 1;
     }
 
-    bool isBayer() const
-    {
-        return (filters != 0 && filters != 9);
-    }
-
     bool isXtrans() const
     {
         return filters == 9;