add misc id24hacked properties

[elf-alchemist]

Implements:

• USER_ mnemonics
• custom item pickup system -- including custom pickup messages, "bonus counts" for the palette flash effect, custom pickup sounds
• customizable nightmare respawn mechanics
• flags for itesm staying on the floor when pickup
• self damage factor

Fixes:

• issue with obituary string assignments in the deh parser

I'm deliberately separating the work for weapon and ammo systems from this PR due to the greater complexity that comes with them, save game compatibility being the most notable thing I have to worry about, but I will target them ASAP.

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Rum And Raisin's teat wad:

Dominus Diabolicus 1's ID24 version, with a custom finale cast call:

[fabiangreffrath]

Can we somehow separate this PR into questionable and no-so-questionable changes or is all of this of-out-question for you @rfomin ?

[rfomin]

Can we somehow separate this PR into questionable and no-so-questionable changes or is all of this of-out-question for you @rfomin ?

I think we should adapt these features to declarate and abandon id24hacked.