Allow trees to be regrown

[Septica]

Since buildinghelper.lua leaves an npc thinker unit on a fallen tree. It blocks the regrowth of the tree. It would be nice to have some way to detect a tree regrowth and destroy the associated npc thinker.

[MNoya]

As there is no tree regrow event, there are 4 default API functions it should need to hook to make this transparent:

• GameRules:SetTreeRegrowTime(time) (Default: 300 seconds)
• GridNav:RegrowAllTrees()
• tree:CutDownRegrowAfter(time, team)
• tree:GrowBack(time)

[MNoya]

Done, give it a try and post again if there's any problem