Improve rain manager script

[Diarandor]

Things to do:

• Make script compatible with scrolling transitions between maps.

• Optimize and clean code to make it faster.

• Do not use movements to avoid engine bug (and make it even faster):
  In my computer, the rain script makes the game to run much slower (with lags) after a few minues (2 or 3 at most). It happens with the engine directly and from the editor too.
  @christopho and others: could you confirm that you have the same problem with this script? Just test it on a big map with the "storm" mode. I don't know what is producing the issue, because the number of drops (Lua lists) is fixed. Maybe it's because my computer overheats after a while, and thus producing the slowness, but I don't know yet, we need to debug on this.

• Fix rain appearing over inventory and similar menus.

[Diarandor]

I confirm, unless I am wrong, that this seems to be an engine bug caused by built-in movements applied to Lua lists. Since my number of lists is constant and their coordinates are replaced by new ones each time they are overriden, that means that the lists or their coordinates are not the problem. So my guess is that:

• either the movements are not being destroyed and they accumulate,
• or some "built-in coordinates" attached to the movement are becoming too big.
  @christopho I will open an engine bug in the correct repo.

[Diarandor]

The rain manager should be now 100% done. All fixed. All suggestions done. ^^