This PR adds Props as a distinct new object type. Props work very similarly to ships in most cases but there are some key assumptions that make them unique:

• Props cannot move
• Props are not interactable except for being collided with

Props are meant to be scenery, set pieces, decoration, or even objects rendered by Lua in the UI only. Props do not take up ship slots and are not limited. Props are created in FRED using the new second dropdown with ctrl-shift-click instead of just ctrl-click

Props have a basic set of flags to start:

• No Collide
• No FRED
• No Lighting

Props have a basic set of sexps to start:

• add-to-collision-group-prop
• remove-from-collision-group-prop
• get-collision-group-prop
• prop-create
• prop-vanish

Props have basic Lua support including collision hooks and table indexing with a standard set of members and methods. Props are also a valid type for Lua SEXPs. Props are viewable in the F3 Lab as well. Props have stubbed in support for @BMagnu animation system and glowpoints but both are inactive for now.

Props.tbl is modular with -prp.tbm. It supports both +nocreate and +remove. Props can be categorized with colors for easy browsing in FRED. A sample table looks like this:

#PROP CATEGORIES

$Name: Planets
+Color: (0, 125, 0)

#PROPS

$Name: Planet-Antares1
	$POF file: planet-antares1.pof
	$Closeup_pos: 0.0, 0.0, -6000
	$Closeup_zoom: 0.5
	$Detail distance: (0)
	$Category: Planets
	$Custom Data:
	+Val: Category Between the Ashes
	$Custom Strings
	$Name: MapText
	+Value: -1
	+String: The first planet of the Antares System.
	$end_custom_strings

#END

Many thanks to @BMagnu and @Baezon for the assistance in various places of this PR, especially with collisions.

Fixes #4727