If you're not a software developer this thread will probably be boring for you...
Anyway. Once upon a time, Glade developers started releasing two branches, 3.8.x and 3.10.x. The first one is intended for GTK+ 2 support (and for making it easier for developers to migrate to GTK+ 3), the second one is for developing modern GTK+ 3 applications. All the details are explained here.
So, the new 3.10.x releases don't include gtk-builder-convert and don't support some widgets which are considered deprecated. If you want to play with some old Glade project you'll have to install 3.8.x alongside 3.10.x. The developers made it possible. Indeed, if you search for "glade" in the Ubuntu repos, you'll find something like "glade-gtk2" or "glade-3" (depending on Ubuntu version I guess). That's 3.8.x branch.
However, there's no such thing in Debian Testing: the proposal to add 3.8.x has been declined. They've included only 3.10.x.
So here's a similar proposal. Bring 3.8.x back. Into LMDE repositories this time.