Fixed mac specific compiler errors and cleaned up Objective C code

Created new file src/gui/macutilities.mm, moved code from mainwindow.cpp and torrentcontentmodel.cpp that used the Objective C runtime into it and converted it to actual Objective C. Rewrote pixmapForExtension() so that it doesn't call into private Qt functions.

src/gui/gui.pri

@@ -121,6 +121,11 @@ win32|macx {
     SOURCES += $$PWD/programupdater.cpp
 }
 
+macx {
+    HEADERS += $$PWD/macutilities.h
+    OBJECTIVE_SOURCES += $$PWD/macutilities.mm
+}
+
 FORMS += \
     $$PWD/mainwindow.ui \
     $$PWD/about.ui \