Fix a bug that shows up in appmenu-firefox

When using appmenu-firefox, the menus (right-click menus, Window menubar's menus, etc.) flicker and sometimes disappear until firefox is restarted.

Here:
https://bugs.launchpad.net/libdbusmenu-qt/+bug/1035755

A patch is proposed, I just applied it to the ebuild and checked that it works now.
It's not upstream yet, however.

I haven't filed a bug in Gentoo, even though their libdbusmenu-qt has the same bug, because you need appmenu-firefox to reproduce it, and it's only in your overlay ATM.
I'm not sure if we should do that, and how. Should I file a Gentoo bug?