FGD cleanup

[SirYodaJedi]

Some notable changes:

• Verify feature parity between Trenchbroom and Jack FGDs
• Move long SmartEdit names to descriptions
• Move relevant key and flag descriptions from entity description to KV description
• Move Trigger baseclass to baseclass section, so that trigger_filter doesn't cause an error
• Remove linebreak statements from trigger_textstory and target_textstory, since it gets interpreted as an actual line break
• Add sequence() and skin() to Jack FGD where appropriate
• Remove several dangerous default values from Jack FGD, as default values are autofilled by the editor
• Add some editor sprites to the Jack FGD (they're bundled with Jack)
• Add sequence variable to defs.qc, because Jack needs it to preview animation frames reverted
• Add some comments indicating where changes would be necessary to use Jack FGD with NRC.

Still working on fixing some syntax errors, but it's 98% done, so I figured I'd push a draft PR to get feedback. This PR is a bit messy, though, so I might need to close it and open a new one if it is to be pushed.

[jjelliott]

I'm fine with doing this, but I was actually thinking of deleting this as I have no interest in maintaining it anymore, so don't be surprised if it gets out of date again.