Don't refresh whole bookmarks-menu on change,

And, limit width of action's title.

Is `qMin(500, QApplication::desktop()->screenGeometry(menu).width() / 4)` a good limit?
Yes, because menu was cleared and a `child` that is of type `Folder` has a menu that is not computed, yet.

*`But I want to know how much this helps? Is it worth the added complexity?`*
Indeed this patch is related to (my) issue with slow icon loading that cause calculating whole bookmarks menu (~1500 bookmarks+~30 folder) needs ~60s to complete!

I think if I solve (and I think that I did) that issue then we can skip this patch because of its complexity. :-)