Add IdleRegister to deal with destruction while idle func is queued

This adds a little helper class to `guiutils.*` that unregisters
in-flight idle functions queued by `IdleRegister::add()`. It's best
to call `IdleRegister::destroy()` in the destructor of the class
owning the `IdleRegister` instance. Otherwise make sure, it is the
last member which will be deleted first.

`Resize` now makes use of this new facility in `setDimensions()`, which
also fixes #3673.

rtgui/guiutils.h

@@ -19,13 +19,19 @@
 #ifndef __GUI_UTILS_
 #define __GUI_UTILS_
 
-#include <gtkmm.h>
-#include <cairomm/cairomm.h>
-#include "../rtengine/rtengine.h"
-#include "../rtengine/coord.h"
-#include "rtimage.h"
-#include <sstream>
 #include <iostream>
+#include <set>
+#include <sstream>
+
+#include <gtkmm.h>
+
+#include <cairomm/cairomm.h>
+
+#include "../rtengine/coord.h"
+#include "../rtengine/noncopyable.h"
+#include "../rtengine/rtengine.h"
+
+#include "rtimage.h"
 
 Glib::ustring escapeHtmlChars(const Glib::ustring &src);
 bool removeIfThere (Gtk::Container* cont, Gtk::Widget* w, bool increference = true);
@@ -40,6 +46,20 @@ void setExpandAlignProperties(Gtk::Widget *widget, bool hExpand, bool vExpand, e
 
 guint add_idle (GSourceFunc function, gpointer data);
 
+class IdleRegister final :
+    public rtengine::NonCopyable
+{
+public:
+    ~IdleRegister();
+
+    void add(GSourceFunc function, gpointer data);
+    void destroy();
+
+private:
+    std::map<void*, guint> ids;
+    MyMutex mutex;
+};
+
 // TODO: The documentation says gdk_threads_enter and gdk_threads_leave should be replaced
 // by g_main_context_invoke(), g_idle_add() and related functions, but this will require more extensive changes.
 // We silence those warnings until then so that we notice the others.