Add option to use system-installed ImGui and ImPlot dependencies

[jeongseok-meta]

Hello, I'm working on adding this package to a package manager (like vsg-dev/VulkanSceneGraph#1375). One issue with it is that vsgImGui uses its dependencies (imgui and implot) via git submodules and assumes specific locations of them when including the dependency headers:

vsgImGui/src/vsgImGui/RenderImGui.cpp

Line 27 in d393985

#include "../imgui/backends/imgui_impl_vulkan.h"

However, building with their dependencies in this way (vendoring) is not recommended, as it can lead to conflicts when the package's dependencies and the vendored ones are different.

I guess the submodule was chosen because it's convenient in many ways, but would you accept PRs that enable vsgImGui to use system-installed dependencies optionally? Here's what I'm thinking of:

option(VSG_IMGUI_USE_SYSTEM_IMGUI "Use system installed ImGui" OFF)
if(VSG_IMGUI_USE_SYSTEM_IMGUI)
  find_package(ImGui CONFIG REQUIRED)
else()
  # the current logic but the header include is the same as when the dependency is installed system-wide
endif()

[robertosfield]

I have been considering dropping the submodule approach now like ImGui and ImPlot are better supported in 3rd party package managers, making it optional would be a good first step.

[jeongseok-meta]

Good to know that! Feel free to review #73 when you have a chance. I've drafted a minimal change for this.