support multiple instances of imgui

[hunt978]

• by switching ImGui context between different renderer instance, now QtImGUI can support multiple instance at the same time. (examples/multiple)
• update source code structure, adjust qmake files but left out testing builds.

~while with a issue that cannot split wheel events among deferent widgets~

[hunt978]

fix wheel event conflicting issues. turns out it's the issue of sharing ImPlot context among widgets.

[seanchas116]

Thank you! The project structure would become much cleaner.