Precompile a handful of Qt headers to speed up compilation time

QGCApplication.pro

@@ -126,6 +126,8 @@ include(QGCExternalLibs.pri)
 # Main QGroundControl portion of project file
 #
 
+PRECOMPILED_HEADER += src/stable_headers.h
+
 RESOURCES += qgroundcontrol.qrc
 
 DEPENDPATH += \
@@ -275,7 +277,8 @@ HEADERS += \
     src/ui/uas/QGCUnconnectedInfoWidget.h \
     src/ui/uas/UASMessageView.h \
     src/MissionItem.h \
-    src/AutoPilotPlugins/PX4/PX4AirframeLoader.h
+    src/AutoPilotPlugins/PX4/PX4AirframeLoader.h \
+    src/stable_headers.h
 
 !iOSBuild {
 HEADERS += \

src/stable_headers.h

+#ifndef STABLE_HEADERS_H
+#define STABLE_HEADERS_H
+
+#include <QtCore>
+#include <QtGui>
+#include <QtWidgets>
+
+#endif // STABLE_HEADERS_H
+