GUI: Convert path to native separators before copying it.

Ticket #2424 (Windows GUI: "Copy full path" doesn't copy the visible full path if it has been changed in preferences)

We keep paths internally with / separator and only convert to native
separators (for Windows) when showing them. Conversion was missing
from path copying function.

gui/resultstree.cpp

@@ -719,7 +719,7 @@ void ResultsTree::CopyPath(QStandardItem *target, bool fullPath)
 
         //Replace (file) with filename
         QString file = data["file"].toString();
-        pathStr = file;
+        pathStr = QDir::toNativeSeparators(file);
         if (!fullPath)
         {
             QFileInfo fi(pathStr);