First round of cleanups for rtengine/dcp.*

- Sorted functions and methods
- Moved local functions to anonymous namespace
- Slightly modernized local functions
- Reworked `DCPStore` and fixed memory leak

Still coming:
- Rework `DCPProfile`

rtengine/init.cc

@@ -42,7 +42,7 @@ int init (const Settings* s, Glib::ustring baseDir, Glib::ustring userSettingsDi
     settings = s;
     iccStore->init (s->iccDirectory, baseDir + "/iccprofiles");
     iccStore->findDefaultMonitorProfile();
-    dcpStore->init (baseDir + "/dcpprofiles");
+    DCPStore::getInstance()->init (baseDir + "/dcpprofiles");
 
     CameraConstantsStore::getInstance ()->init (baseDir, userSettingsDir);
     profileStore.init ();