Update Dear ImGui to v1.91

[lshoek]

• Updates Dear ImGui to version 1.91.9b. I deliberately chose this version as 1.92 introduces some large changes to the rendering backends and fonts.
• Adapted napimgui and nap::ImGuiService to deal with the increased responsibility over input management on the ImGui side.

There are a few breaking changes in the ImGui API, but they are well documented. Instances of commonly used functions include:

• ImGui::SliderFloat
• ImGui::InputFloat
• ImGui::GetKeyIndex

[cklosters]

So what's the actual reason to push this? Are there any new features you really need?

[lshoek]

The big one for me is the feature-rich tables API. Very powerful for creating monitors and control panels for lots of elements/devices/agents etc. The previous columns API was rather clunky. There are also options to view-clip large lists. Other than that I've been inconvenienced more frequently by small quirks and issues that were addressed in later versions.

[cklosters]

Thanks for the clarification, makes sense; I'm currently working on a lot of GUI specific code, probably good to merge this change into NAP and use that as a base instead of our current version of ImGUI, but that won't be immediately unfortunately because I'm rather busy getting some other things off the ground.