core: add class that collects global objects to be deleted on exit

We have a prevailing problem with global QObjects defined as
static global variables. These get destructed after main()
exits, which means that the QApplication object does not
exist anymore. This more often than not leads to crashes.

In a quick search I didn't find a mechanism to register
objects for deletion with QApplication. Therefore, let's
do our own list of global objects that get destructed
before destroying the QApplication.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

subsurface-helper.cpp

@@ -8,6 +8,7 @@
 #endif
 
 #include "stats/statsview.h"
+#include "core/globals.h"
 #include "core/qt-gui.h"
 #include "core/settings/qPref.h"
 #include "core/ssrf.h"
@@ -67,6 +68,7 @@ void exit_ui()
 #ifndef SUBSURFACE_MOBILE
 	delete MainWindow::instance();
 #endif // SUBSURFACE_MOBILE
+	free_globals();
 	free((void *)existing_filename);
 }