add a way to keep certain widgets inside of visible area

Nobody has claimed this yet.

When talking about _"screen area"_, what we really have in mind is "_the backbuffer that Nuklear is drawing to_". Not to be confused with nk_window or the physical display! (I feel like I only made it more confusing now...)

Follow up from https://github.com/Immediate-Mode-UI/Nuklear/pull/881#discussion_r2746144225

> > It also would be really nice for the tooltip to stay in the available screen space. Right now it just draws outside so you can't see its contents [screenshot below], however, I'm not sure if this problem is solvable right now (like does Nuklear keep the OS Window size somewhere?)
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> I've thought about the offscreen issue too. I'm sure there's a way but whether it's pretty or backend specific or not is the question. I'll look into it after I finish the rest of it.

^ There are at least few widgets that would benefit from this: definitely Tooltips and MenuBar Items, and probably other stuff based on Popups. I'm also planning to implement "_cascaded menus_" in the future, and those will need some form of screen awareness. In theory: every backend should be able to support this.

Implementation note: some backends can also provide a detailed info about which part of the screen is safe to display something meaningful (e.g. [SDL_GetWindowSafeArea](https://wiki.libsdl.org/SDL3/SDL_GetWindowSafeArea)) so it would be nice to take this into consideration.

Start with the tooltip, menu-bar item, and other popup behavior described in the issue, then review how the backends expose the drawing area and optional safe-area information. Define how widgets should remain within the Nuklear backbuffer, including cascaded menus, and validate the behavior across supported backends.

Written by the indexing model from the issue text.