Random items mutator and item system revamp

This mutator randomly replaces items on the map and adds random loot from killing players. Detailed description is here.

Unfortunately, the code didn't have proper support for spawning items so TimePath and I had to refactor a lot.

New stuff

• m_canonical_spawnfunc - weird name. Specifies the classname which will be forced for that item. So all items now obey it.
• m_spawnfunc_hookreplace - Function that is used to replace item definitions.
• SPAWNFUNC_ITEM - new way to define spawnfunc for items. Calls StartItem with specified item definition.
• SPAWNFUNC_WEAPON - new way to define spawnfunc for weapons. Calls weapon_defaultspawnfunc with specified item definition.
• ITEM_FLAG_* enum.
• ITEM_FLAG_NORMAL - indicates that item is usable during normal gameplay. Some items, such as new toys, have this flag too.
• ITEM_FLAG_INSTAGIB - indicates that item is usable during instagib.
• ITEM_FLAG_OVERKILL - indicates that item is usable during overkill.
• ITEM_FLAG_MUTATORBLOCKED - if set, items with this flag will not be spawned.
• Item_Create - spawns a new map item.
• Item_Initialize - initializes already spawned entity as item.
• Item_CreateLoot - creates loot.
• Item_InitializeLoot - initializes already spawned entity as loot.
• Item_IsLoot - checks if item is loot. Replaces old droppedweapon check.
• Item_SetLoot - sets the item loot status.
• Item_IsExpiring - checks if item is expiring, i.e. it's superweapon, strength and shield time ticking while it is on the ground.
• Item_SetExpiring - sets item expiring status.
• ItemTouched hook - called after item has been touched.
• m_isreplaced - holds whether weapon has been replaced. Replaces old replacedweapon check.

Item renaming

• Devastator now has weapon_devastator classname.
• HLAC now has weapon_hlac classname.
• Mine Layer now has weapon_minelayer classname.
• Mortar now has weapon_mortar classname.
• Rifle now has weapon_rifle classname.
• TAG Seeker now has weapon_seeker classname.
• Vortex now has weapon_vortex classname.
• Shield now has item_shield classname.
• droppedweapon classname was completely removed.
• replacedweapon classname was completely removed.

Instagib

• g_instagib_invisibility_time - time in seconds of the invisibility powerup.
• g_instagib_speed_time - time in seconds of the speed powerup.
• Vaporizer cells now have item_vaporizer_cells classname.
• Extra life now has item_extralife classname.
• Invisibility now has item_invisibility classname.
• Speed now has item_speed classname.
• Invisibility and Speed now glow and have correct sounds as their vanilla counterparts.

Overkill

• Overkill now uses ITEM_FLAG_MUTATORBLOCKED to disable items.
• Loot from dead players uses new loot API.