importtool: add "don't ask again" option on "no active sequence" dialog

In the previous commit, we added a dialog to give users options when dropping
footage on a sequence-less timeline panel. However some users will have a
preferred behavior that they'd want every time without having to go through
the messagebox, so we add a "don't ask again" option that stores the setting
in the application config.

app/config/config.cpp

@@ -75,6 +75,7 @@ void Config::SetDefaults()
   config_map_["Autoscroll"] = AutoScroll::kPage;
   config_map_["DefaultViewerDivider"] = 2;
   config_map_["AutoSelectDivider"] = false;
+  config_map_["DropWithoutSequenceBehavior"] = TimelineWidget::kDWSAsk;
 
   config_map_["DiskCachePath"] = QStandardPaths::writableLocation(QStandardPaths::AppLocalDataLocation);
   config_map_["DiskCacheSize"] = 20.0;