Fully support reverse selection mode on selection tools

[curiosity]

Right now we have full means to work with normal and alternative selection modes. The reverse selection mode can be defined in the selection tool prototype and invoked from code (LuaPlayer::start_selection), but there is no (non-exploit) way for the player to use it and no event for it. I ask for these to be added so that the reverse selection mode can be fully supported and usable by mods. It adds a potential third selection type to the tool, which I'm sure some mod authors will find handy.

[NotRexButCaesar]

I would like to also request this: having three modes for a selection tool would be useful for me.

[curiosity]

Implemented in 1.1.58.