Introduce v1.5 compatibility

[mfrancis95]

Probably the most pointless version "emulation" you'll ever see. Based on https://doomwiki.org/wiki/Versions_of_Doom_and_Doom_II#v1.5:

• Introduced a new exe_doom_1_5 game version.
• "Factored in mobj radius in P_CheckMeleeRange" - used exe_doom_1_5 as a comparison point instead of exe_doom_1_2.
• "Replaced short-range hitscan by melee range check in A_SargAttack" - same as the above.
• "Monster barrel suicides are no longer possible" - same as the above.

So in reality, not much was actually changed and this is mostly semantics, but this is to also help pave the way for inserting 1.4 compatibility in between which has its own changes distinct from 1.2 and 1.5.

To Do:

• Figure out what the demo identifier for 1.5 is and auto-detect it so the -gameversion doesn't always need to be explicitly passed.
• Find out what "Deathmatch bug fixed where items reappeared but could not be picked up" is about and see if this can be addressed or matched up to this new version in the code.

[turol]

Looks reasonable but I have not actually tested any of it.