Minor improvements & fixes to music player

- music will be selected correctly after rapid leaving and reentering
town screen
- music sets are now store as single std::map instead of unnecessary
"map of maps"
- fixed computation of resume time for restarted/looped music tracks
- updated & clarified some comments
- converted C functions for SDL_Mixer callbacks into lambdas
- formatting fixes

lib/VCMIDirs.cpp

@@ -170,6 +170,8 @@ class VCMIDirsWIN32 final : public IVCMIDirs
 
 void VCMIDirsWIN32::init()
 {
+	std::locale::global(boost::locale::generator().generate("en_US.UTF-8"));
+
 	// Call base (init dirs)
 	IVCMIDirs::init();
 
@@ -698,11 +700,6 @@ namespace VCMIDirs
 		static bool initialized = false;
 		if (!initialized)
 		{
-			#ifdef VCMI_WINDOWS
-			std::locale::global(boost::locale::generator().generate("en_US.UTF-8"));
-			#endif
-			boost::filesystem::path::imbue(std::locale());
-
 			singleton.init();
 			initialized = true;
 		}
@@ -710,5 +707,4 @@ namespace VCMIDirs
 	}
 }
 
-
 VCMI_LIB_NAMESPACE_END