makefile: add update rule

[gucio321]

Intro

cimgui-go needs a simple way to update its dependencies (since it is a basic premise of the project).
This Pull Request introduces make update command.

Changelog

• put all imgui dependencies in one folder (currently called cimgui)
• change structure of imgui dependencies directory (now it consists of cimgui{_impl}.(h/cpp), imgui/ upstream and cimgui_templates with json defs)
• use custom CMakeList.txt (this one in libs folder) which builds cimgui/cimplot shared library (note I think we might want to add glfw as well and combine everything in one shared lib)
• update code

Issues and reference

• "successor" of Cimgui updater #100
• cimgui generator with extra args (-DIMGUI_USE_WCHAR32) can't make it use ImWchar32 cimgui/cimgui#232

[gucio321]

it works now, finally!!!
Let it make it all together and it'll be ready!

[gucio321]

@AllenDang, @eliasdaler if tests passes (I hope they will), this PR is ready to be reviewed!

[AllenDang]

LGTM