misc: remove code duplication and use native theme on KDE

GitLab: #603
Change-Id: I271f6d4b9b8089dc88479f1d7d98514acb3d52be

src/main.cpp

@@ -62,7 +62,9 @@ main(int argc, char* argv[])
     QList<char*> qtWebEngineChromiumFlags;
 
 #ifdef Q_OS_LINUX
-    setenv("QT_QPA_PLATFORMTHEME", "gtk3", true);
+    if (!getenv("QT_QPA_PLATFORMTHEME")
+        && !(getenv("XDG_CURRENT_DESKTOP") == "KDE" || getenv("XDG_CURRENT_DESKTOP") == "GNOME"))
+        setenv("QT_QPA_PLATFORMTHEME", "gtk3", true);
     setenv("QML_DISABLE_DISK_CACHE", "1", true);
 
     /*

src/mainapplication.cpp

@@ -165,11 +165,6 @@ MainApplication::init()
 
     setWindowIcon(QIcon(":/images/jami.ico"));
 
-#ifdef Q_OS_LINUX
-    if (!getenv("QT_QPA_PLATFORMTHEME"))
-        setenv("QT_QPA_PLATFORMTHEME", "gtk3", true);
-#endif
-
     if (runOptions_[Option::Debug].toBool()) {
         consoleDebug();
     }