dfmanager cleanup (#6211)

* Turn `DFManager` into a singleton
* PIMPL `DFManager`
* Cleanup namespace usage in `dfmanager.cc`
* Constify `DFManager` interface
* Fix bad `reinterpret_cast` between `std::string` and `Glib::ustring`

Co-authored-by: Flössie <floessie.mail@gmail.com>
Co-authored-by: Thanatomanic <6567747+Thanatomanic@users.noreply.github.com>

rtgui/toolpanelcoord.h

@@ -307,7 +307,7 @@ public:
     }
 
     //DFProvider interface
-    rtengine::RawImage* getDF() override;
+    const rtengine::RawImage* getDF() override;
 
     //FFProvider interface
     rtengine::RawImage* getFF() override;