Provide keyboard/mouse/touch-pad/touch-screen instructions

[EricBoix]

Provide a widget (this could complement the about button?) that graphically and synthetically
documents the usages of the keyboard, mouse, touch-pad, touch-screen when interaction with
an UD-Viz scene.

In the keyboard section of that GUI documentation, don't forget to document the "point-and-space-bar"
feature.

Leave it to the application to start with this documentation displayed (requiring a action to close the sub-window) or not (just as the About does it)..

Notes

Interaction modes: fly-mode vs game-mode

Because UD-Viz blends two categories of users, they are probably implicitly two interactions modes:

• a (kind of) fly mode: only the mouse is used to go forth and back in the scene (following the
  axis of of the camera) and turning around (the camera)
• a game mode where
  • keyboard decides where to move (forth/back and sideways), jump, dodge ,run...
  • the mouse decides where to look at (direction of the camera)

Document the existence of those modes, offer a mode-toggle key and document everything.

Inspiration sources

UD-Viz documentation already has the following picture in docs/static/Doc/User/Pictures/UserDoc/controls.PNG

The following image source was lost (and might be copyrighted): take some inspiration but do not import it as-is within UD-Viz:

.