Fix Window->Multimeter submenu being always empty

[helaslo]

While developing a driver a multimeter, I noticed that I wasn't able to re-open the window for it after closing it because the menu looked only for scopes, not DMMs, this fixes it.

[azonenberg]

I'll keep this open for the near term but most likely will not be merging. The long term plan is to have meters controlled via the stream browser and filter graph like power supplies, rather than in a complex sea of popup windows (this is the case for most other instruments too).