GTK: run DoLater after the next main loop iteration.

Before this commit, DoLater would be run as an idle callback,
which (depending on system performance) could either result in
a half-regenerated sketch being displayed, with only the dragged
entity updated, or no regeneration whatsoever during the drag.

After this commit, the GTK behavior matches macOS and Win32 ones.

src/platform/gtkmain.cpp

@@ -213,7 +213,7 @@ static bool LaterCallback() {
 }
 
 void ScheduleLater() {
-    Glib::signal_idle().connect(&LaterCallback);
+    Glib::signal_timeout().connect(&LaterCallback, 0);
 }
 
 /* Editor overlay */