Include of Dear ImGui library in the ImPlot

[DDeimos]

Hello! There are two includes of Dear ImGui headers:

1. imgui.h in the implot.h
2. imgui_internal.h in the implot_internal.h

Both headers are included as it is without extra imgui directory. Usually, libraries expect that they will be included with specific root directory. Here are some libraries, for instance:

• imgui/imgui.h
• box2d/box2d.h
• glm/glm.h

This allows you to setup some lib/thirdparty/extern common directory to work with. You don't have to specify path to each library explicitly. On the other hand, include without root directory forces users to set global access to the imgui directory for each platform and for each project to make it visible for ImPlot. Is there any specific reason why ImPlot expects to see imgui.h locally and not as imgui/imgui.h?