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/imagesource.h

@@ -137,6 +137,7 @@ public:
 
     virtual ImageMatrices* getImageMatrices () = 0;
     virtual bool           isRAW () const = 0;
+    virtual bool           isGainMapSupported () const = 0;
     virtual DCPProfile*    getDCP (const procparams::ColorManagementParams &cmp, DCPProfileApplyState &as)
     {
         return nullptr;