Tweaks for Gravity Mode

The following discussion from !112 (merged) should be addressed:

• @kartmann started a discussion:

  • When clicking the Z orientation indicator, the "gravity" axis seems to change. It behaves similarly, but the fixed axis changed. You can recover by clicking on X or Y, which makes Z the gravity axis again.
  • When fixed gravity mode is active, Roll should not be displayed in the control ads (both technical and playful)

Plus:

• When in orthographic + playful mode, rename "Up" and "Down" (Space/X) in control ads to "Zoom in / Zoom out"