Provide length of LuaTransportLine

[Atria]

LuaTransportLine allows placing items on the line but doesn't provide range of the "position" argument. We can check if item can be inserted to specific position, but when it fails we don't know if it failed because of another item blocking the position or that I tried to place item after then end of the line.

Suggestion: Provide new attribute which gives highest valid value of "position" (with 0 being expected lowest value). Basically a length of the transport line.

[boskid]

This is implemented for 2.0 as LuaTransportLine::line_length.