Support multiple monitors better

Nobody has claimed this yet.

As reported by @ehrenfeu:

> I'm working with a dual-screen layout (internal notebook + external TFT), where the external one is a lot bigger and thus naturally my primary screen.
>
> Depending on the configuration where Unity should place its launcher, ImageJ places _most_ of its windows on the same screen (e.g. the main window, newly opened files, brightness&contrast). However, some windows always get placed on the leftmost screen (histogram, action bars, ...), which is a bit confusing.
>
> So apparently some windows follow a "preferred" screen policy while others stubbornly stick to the first available screen.

We could fix this at the `ij-legacy` level by adding a patch to be smarter about using `GraphicsEnvironment` to show dialogs on the expected screen in more cases.

Start in the ij-legacy level and locate the window and dialog placement code that uses GraphicsEnvironment. Reproduce the reported dual-screen behavior with a larger external primary display and compare the affected windows. Done means the histogram, action bars, and other dialogs follow the expected preferred-screen policy instead of consistently using the leftmost screen.

Written by the indexing model from the issue text.