Add basic map backup system

Makes backups of maps on a per-archive/per-map basis, currently only on save or map open (might add background saving of backups on a timer at some point). Also might be extended to all archive entries in the future
Fix map entries saving incorrectly when saving a new map twice
Line UDMF transparency should be 1.0 by default, not 0
Use console output instead of acs.err if the file does not exist after compiling acs with errors
Bump version to 3.1.0

Also update installer script to download and install VS2013 runtimes if needed
Merge branch 'master' of https://github.com/sirjuddington/SLADE into osx