Gravity control: Off, Hyperbolic, Realistic (inv. squared) etc.

[xparq]

Mainly to support #65. (See also the gravity strength slider: #175.)

[xparq]

OK, the weird old Skewed mode made no sense after all (being based on a misunderstanding (-> #65) to begin with...), so that value (i.e.: 2, possibly surviving in various saved states) has been

• [x] reinterpreted as Hyperbolic, and now (also) used for (different) testing. (Will probably become the default hyperbolic mode in the future!)
  • [x] Renaming to Experimental was also considered (with "officially undefined" behavior), but rejected for the unresolvable confusion it would cause between different snapshot versions, i.e. the inability to load old versions with the same old (experimental) behavior ever again. -> Well, this rejection also means that it has just been given a name, to pretend it's not experimental, and that it will be retained in the code for as long as that old mode value will be supported... The issue being that the old Normal mode (= 1) is effectively equivalent with this new one, but slower... Both mean the same, but only one should survive -- and that should be the other (old) one, with many more existing saved snapshots... -> Saving/loading experimental properties should still be supported, but without any compatibility guarantees between versions! (#529)
• [x] Also added a Realistic option (which doesn't really work yet: #525).