Fix initialisation of the global logging category variable

Just accessing the raw variable doesn't guarantee the order
of construction and may lead to crashes in certain constellations.

libnymea-core/nymeacore.cpp

@@ -601,7 +601,7 @@ QStringList NymeaCore::getAvailableLanguages()
 
 QStringList NymeaCore::loggingFilters()
 {
-    return s_nymeaLoggingCategories;
+    return nymeaLoggingCategories();
 }
 
 QStringList NymeaCore::loggingFiltersPlugins()

libnymea/integrations/integrationplugin.cpp

@@ -100,6 +100,9 @@
 /*! IntegrationPlugin constructor. IntegrationPlugins will be instantiated by the system.
     This should never be called manually by a plugin implementation.
 */
+
+NYMEA_LOGGING_CATEGORY(dcIntegrations, "Integrations")
+
 IntegrationPlugin::IntegrationPlugin(QObject *parent):
     QObject(parent)
 {

libnymea/loggingcategories.cpp

@@ -34,10 +34,12 @@
 #include <QDateTime>
 #include <QMutex>
 
-QStringList s_nymeaLoggingCategories;
+QStringList& nymeaLoggingCategories() {
+    static QStringList _nymeaLoggingCategories;
+    return _nymeaLoggingCategories;
+}
 
 // FIXME: Those should eventually disappear from here
-NYMEA_LOGGING_CATEGORY(dcIntegrations, "Integrations");
 NYMEA_LOGGING_CATEGORY(dcThing, "Thing")
 NYMEA_LOGGING_CATEGORY(dcThingManager, "ThingManager")
 NYMEA_LOGGING_CATEGORY(dcSystem, "System")

libnymea/loggingcategories.h

@@ -34,13 +34,12 @@
 #include <QLoggingCategory>
 #include <QDebug>
 
-extern QStringList s_nymeaLoggingCategories;
-
+QStringList& nymeaLoggingCategories();
 
 #define NYMEA_LOGGING_CATEGORY(name, string) \
     class NymeaLoggingCategory##name: public QLoggingCategory { \
     public: \
-    NymeaLoggingCategory##name(): QLoggingCategory(string) { s_nymeaLoggingCategories.append(string); } \
+    NymeaLoggingCategory##name(): QLoggingCategory(string) { nymeaLoggingCategories().append(string); } \
     }; \
     static NymeaLoggingCategory##name s_##name; \
     const QLoggingCategory &name() \