fix(Qt6): FTBFS with Qt 6

1. QMake project file should use dtk6 when build with Qt 6
2. FollowRedirects attr for QNetworkRequest is on by default on Qt 6
3. QLabel::pixmap() always return a QPixmap instead of a const pointer

src/spark-store.pro

@@ -31,7 +31,8 @@ DEFINES += APP_BRANCH=\\\"'$$system(git symbolic-ref --short -q HEAD)'\\\"
 # We want to capture all logs (INFO, DEBUG, WARNING, ERROR) for export to /tmp/spark-store
 
 CONFIG += c++11 link_pkgconfig
-PKGCONFIG += dtkcore dtkgui dtkwidget
+equals(QT_MAJOR_VERSION, 6): PKGCONFIG += dtk6core dtk6gui dtk6widget
+equals(QT_MAJOR_VERSION, 5): PKGCONFIG += dtkcore dtkgui dtkwidget
 
 SOURCES += \
         backend/DataCollectorAndUploader.cpp \