Multi-Sided Sign Support (#2796)
* Added SignContentsBack ItemTag property and LocationTag tag and mechanism.
Added back-of-sign support to CopyBlock command
* slight corrections
* one more
* slight adjustments
* meta removal fix from merge
* meta fix from original pr implementation
* added back-of-sign glowing and glow color tags and mechanisms
* minor changes
* renamed `sign_x_back` usages to `sign_back_x`
* missed one
* meta clarification
* minor changes
* fixes
* import changes
(
commit: 118a5b0)
