Base: Move App::Color to Base

Every basic data type is stored in Base module, color is standing out as
one that does not. Moving it to Base opens possibilities to integrate it
better with the rest of FreeCAD.

src/Mod/Start/Gui/StartView.cpp

@@ -136,9 +136,9 @@ public:
             "User parameter:BaseApp/Preferences/Mod/Start");
 
         auto getUserColor = [&hGrp](QColor color, const char* parameter) {
-            uint32_t packed = App::Color::asPackedRGB<QColor>(color);
+            uint32_t packed = Base::Color::asPackedRGB<QColor>(color);
             packed = hGrp->GetUnsigned(parameter, packed);
-            color = App::Color::fromPackedRGB<QColor>(packed);
+            color = Base::Color::fromPackedRGB<QColor>(packed);
             return color;
         };