2.9.0-alpha1 - provide the bed selector to the plate view too

[EricPobot]

The new multiple beds feature is awesome and I enjoy it a lot. I appreciate the fact that I can now bundle a project as a single 3mf file, instead of creating one for each part or group of parts.

However, navigating from bed to bed in the plate view tends to be a bit uncomfortable sometimes, especially if the project uses a lot of parts scattered among a lot of beds.

It would be nice to have the same bed selector as in the slicing view there. In addition to the individual beds choices, it would also have a "global view" choice for showing all the beds, as it works currently. When a bed is selected, only this one is displayed in the plate view, as if the project were a single bed style one.