qt53d: enable assimpsceneparser plugin

- add hint to help text
- add assimp dependency
- always install the gltf (and any future) sceneparser to target.

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas: update commit message as suggested by Arnout.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>