Reformat codebase to be more consistent with the style used in ImGui

[Nelarius]

This codebase grew out of a different project of mine and, unfortunately, I didn't fully transform it to conform to ImGui's style at an early stage -- so to this day, an entirely different coding style is used internally. I figured a full transformation all at once would hurt a little less than frequent incremental changes, so this pr includes some changes to imnodes.h as well. As a result, the code should be a bit easier to integrate with C.

So just a heads up @sonoro1234 and @hoffstadt that this change is incoming since this might affect your imnodes integration work 👀

• [x] all internal functions to CamelCase
• [x] all members, internal and external, to CamelCase
• [x] static constants to UPPER_CASE
• [x] struct and enum definitions moved out of the imnodes namespace and prefixed with ImNodes
• [x] enum constants prefixed with ImNodes
• [x] namespace to CamelCase: ImNodes
• [x] update imnodes.h comments
• [x] update README.md

[hoffstadt]

Thanks for the heads up! And we appreciate your work on this.

[sonoro1234]

Sorry for not realizing this earlier: namespace ImNodes will collide with https://github.com/rokups/ImNodes/blob/02b085dace8bdd815374d6440efb1f70d8a9ebaf/ImNodes.h#L35