Move theme detection into Application

* Add function to Application to quickly determine if in light or dark theme
* Add kpxcApp symbol
* Explicitly define main function for GUI tests to improve performance and use custom Application.

src/gui/Application.h

@@ -41,6 +41,7 @@ public:
 
     bool event(QEvent* event) override;
     bool isAlreadyRunning() const;
+    bool isDarkTheme() const;
 
     bool sendFileNamesToRunningInstance(const QStringList& fileNames);
 
@@ -68,6 +69,7 @@ private:
     static int unixSignalSocket[2];
 #endif
     bool m_alreadyRunning;
+    bool m_darkTheme = false;
     QLockFile* m_lockFile;
     QLocalServer m_lockServer;
     QString m_socketName;
@@ -76,4 +78,6 @@ private:
 #endif
 };
 
+#define kpxcApp qobject_cast<Application*>(Application::instance())
+
 #endif // KEEPASSX_APPLICATION_H