VS: Add a VS_TOOL_OVERRIDE source file property

This property allow to specify a specific Visual Studio tool for a
source file overriding the default tool behavior.  For example, a
`.resw` file being processed as a `PriResource` file.  This has the
advantage of being able to teach CMake to process new file types without
code modifications.

Source/cmVisualStudio10TargetGenerator.cxx

@@ -1203,6 +1203,11 @@ void cmVisualStudio10TargetGenerator::WriteExtraSource(cmSourceFile const* sf)
     }
   }
 
+  const char* toolOverride = sf->GetProperty("VS_TOOL_OVERRIDE");
+  if (toolOverride && *toolOverride) {
+    tool = toolOverride;
+  }
+
   std::string deployContent;
   std::string deployLocation;
   if (this->GlobalGenerator->TargetsWindowsPhone() ||