Move to next stageable line after adding a line to a custom patch

While it's true that the behavior is a little different from the staging panel,
where the staged lines are actually removed from the view and in many cases the
selection stays more or less in the same place, it is still very useful to move
to the next stageable thing in the custom patch building view too.

pkg/gui/controllers/patch_building_controller.go

@@ -155,6 +155,8 @@ func (self *PatchBuildingController) toggleSelection() error {
 		state.SetLineSelectMode()
 	}
 
+	state.SelectNextStageableLine()
+
 	return nil
 }