Stats compatibility with A-Frame 1.8.0+

[diarmidmackenzie]

Work-in-progress PR for A-Frame stats changes coming in 1.8.0 as per:
aframevr/aframe#5700

Basically working, just 2 minor issues:

• conditional includion of the CSS depends on A-Frame version string being 1.8.0+, which isn't yet the case in A-Frame master.
• reason we don't want to include the new CSS with A-Frame 1.7.0 and earlier is that the replacement CSS moves the stats panel down 48px to make way for the new stats view. We'd prefer to keep the old behaviour with A-Frame 1.7.0 and before.
• one minor point with A-Frame 1.8.0, the stats will be 48px lower even if the new stats panel isn't displayed. That's not ideal but a little fiddly to solve, so I don't yet have a solution for that.

I'm undecided whether to merge this now, or wait until A-Frame 1.8.0 is available and I can actually test with that. But either way it's pretty much ready to go when needed.