ZScript: moved actor inheritance code to separate step; ZScript: ported game configuration inheritance code from DECORATE

Fixed null reference access while searching for superclass state; Fixed incorrect parsing of random() in frame duration; Fixed incorrect parsing of quoted sprite names

Internal: started implementing ZScript parser. Nothing works for now, aside from the actual parsing code.

fixed: Texture Select: visible items were not clipped correctly;
fixed: 128x128 texture previews were too large for small window size in Texture Set editor

Updated, Textures Browser: texture name length is now taken into account when calculating the width of texture items.
Internal: moved thing helper shapes creation logic from ThingsMode to LinksCollector.

* Updated the EE config linedefs, fixing stuff around
* Added back the class definitions for the Eternity namespace
The only namespaces where they're unneeded are the vanilla games. Otherwise it's safe and important to keep class1, class2, class3 in case Hexen pops up
* Added new special (273) to UDMF Eternity linedefs
* Added special 140 to the Eternity linedefs