Add ImGui support

[Meakk]

Add a new F3D_MODULE_UI enabling rendering UI widgets using ImGui.
Only the core library of imgui is used and a home-made VTK backend is included in this PR.

For now, only the filename widget is ported to this new framework.

[Meakk]

Current status:

imguidemo.mp4

[codecov]

Codecov Report

Attention: Patch coverage is 71.94093% with 133 lines in your changes missing coverage. Please review.

Project coverage is 95.69%. Comparing base (f3d7627) to head (1e5742a).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
vtkext/private/module/vtkF3DImguiObserver.cxx	41.01%	128 Missing ⚠️
vtkext/private/module/vtkF3DUIActor.h	40.00%	3 Missing ⚠️
vtkext/private/module/vtkF3DUIObserver.h	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1077      +/-   ##
==========================================
- Coverage   96.97%   95.69%   -1.29%     
==========================================
  Files         110      118       +8     
  Lines        8872     9319     +447     
==========================================
+ Hits         8604     8918     +314     
- Misses        268      401     +133     

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[mwestphal]

some changes needed

[mwestphal]

small changes needed, also I still think that doing the superbuild work in advance would be worth it to avoid nightly disruption

[mwestphal]

1 year after PR creation, nice :)