Improve UI using ImGui integration

[mwestphal]

Context
Since F3D 3.0, F3D now integrate ImGui has the main UI framework. It is still in early stages and there is much more to do:

Improve existing UI widgets:

1. Global Improvements:

• Rework UI/UX using designer inputs #1976
• Default font does not support Unicode #1016
• Convert dropzone widget into ImGui widget #1975
• Add model/material/hierarchy and information #18

2. Console:

• ImGui minimal console #1817
• ImGui console scrollbar is not grabbable with the mouse #1785
• ImGui console should support auto completion #1782
• ImGui console should support "Up"/"Down" to circle through inputted commands #1781

3. Cheatsheet:

• ImGui Cheatsheet is not vertically centered #1780
• Improve cheatsheet #29