Urgent: Moving away from imgui package

[Tremeschin]

The imgui package on PyPI seems on the edge of being abandoned or having slow release cycles, with other alternatives listed being closely on the same boat, but perhaps not imgui_bundle.

This is the current main blocking factor of moving away from Python 3.11.

There aren't wheels for CPython 3.12+, making it worse the installation experience on needing compilers and/or failing on 32 bit Python interpreters that people occasionally, and without knowing it, use.