Development work for level_graph, CExplosive, CGrenade, CWeaponStatMgun, CWeapon, script mutant movement. Export CGrenade functions.

[GhenTuong]

level_graph:
LUA: level.get_nearby_vertices(...) can be used to collect nearby vertices.
LUA: level.vertex_link(...) can be used to get the vertices that is connected to the current vertex.
LUA: level.get_nearby_covers(...) can be used to collect nearby vertices that are covers.

CExplosive, CGrenade:
Unify changes under #define EXPLOSIVE_CHANGE.
"on_grenade_explode" can be added to grenade configuration section to fire a callback when the grenade explode.
"explode_on_contact" can be added to grenade configuration section to make the grenade explode when it lands on a solid surface.
Export explosive object and grenade get/set functions.

CWeaponStatMgun:
Set Actor position when leaving machine gun turret.

CWeapon:
Export:
Get_mOffset
Get_mStrapOffset
Get_strap_bone0
Get_strap_bone1
Get_strapped_mode
HandDependence

Script mutant movement:
Allow the use of move.walk_fwd and move.run_fwd since they're also legit movement flags.

scripts\xr_combat_ignore.script
Add a fix to properly set enemy_id to nil when the enemy is no longer valid.