ImGui: Prevent abuse of built-in elements by user code

[ProfLander]

Mods are beginning to emerge which make use of ImGui, but not the grouping API designed for well-behaved sharing of widget space: #354

If left unaddressed, modlists with many such scripts are going to end up with more entries than horizontal space, rendering a subset of them inaccessible.

Example: Simple Task Manager

This also extends to the other 'root' UI elements, like the Main widget which contains the menu; for instance, we might want to inject invisible docking zones for the sides and center to simplify window management, but user code is currently at liberty to interfere.

Summary of existing suggestions:

• Block access to reserved elements at the Lua level (@ravenascendant)
• Restrict manipulation of reserved elements to C++ (@xr-lucy)
• Monkey-patch misbehaving mods to force their menu into a more appropriate place (Catspaw)

[ProfLander]

Personally I favor obfuscation via system-level LTX / Lua over sandboxing into C++, since API access is already an ongoing problem for Stalker scripting.

Admittedly, this is an API we don't really want to be public, but keeping it in the engine's simpler languages reduces maintenance burden; as an example, something like fsgame.ltx appears too scary / arcane / undocumented for most scripters to touch, while making changes relatively easy for maintainers.