Floorbot upgrade + general tweaks

- Bridge mode has returned! Set the direction, and the bot shall build
you a bridge through space!

- "Improve floors" is now named "Replace tiles", for clarity.

- Floor and cleanbots will now keep a list of targets to ignore, so that
unreachable targets no longer effectively shut them down.

- Moved some things from turn_off() procs to bot_reset() procs

- Changed "BOT_WORKING" to "BOT_MOVING" to better clarify what the bot
is doing.

- Reworked floorbot logic a bit.