Add Cordon support to VMFLib, & hidden solids

Won't be needed for the PeTI, but it's nice to have for use when using
the library on actual Hammer maps.
Fix some bugs with VMFLib

- Cameras crash it
- Special handling to allow "solid" to be used as a keyvalue name
- Change output parsing to allow having both , and chr 27 in the string
(chr 27 is the separator, this lets commas appear in the values.)