Hopefully fixed a race condition that could occur when pressing "Print" (file not opened yet, but comm layer tries to fetch next command)
Changed handling of bed temperatures

Will now be left unset if not detected (instead of dying a horrible death), modified frontend to not display bed settings in such cases. While at it also (hopefully) fixed "Target: off" for bed issue.

TODO: Support repetier's "TargetBed:", "TargetExtr%n" syntax

Fixes #399, partially solves #360