Fix various XCode and PVS-Studio static analyzer warnings (#2309)

Renamed stb_truetype.h to imstb_truetype.h, stb_textedit.h to imstb_textedit.h, and stb_rect_pack.h to imstb_rectpack.h. (#1718, #2036) ...

If you were conveniently using the imgui copy of those STB headers in your project, you will have to update your include paths.
The reason for this change is to avoid conflicts for projects that may also be importing their own copy of the STB libraries. Note that imgui's copy of stb_textedit.h is modified.