Provide a unified libimgui

[greaka]

Currently the greatest argument to support arcdps over al is the enforced and stable imgui backend. while lib_imgui provides similar utility, it is an extra dependency that isn't treated as first party. The arcdps way of setting up imgui is simple and well documented and allows for connecting to and interacting with all other windows from arc or plugins. a good example is the window snapping utility of the arcdps extension template from knox

[megai2]

I can't add imgui context in core of loader.
Extra dependencies should be handled by manager & addon description.

Right now sample addon uses lib_imgui, that promotes it as a "frist party" thingy,
but manager does not handle dependency at all and this is what should be worked in my opinion.