6B3 variants

Added 5 new variants of Soviet 6B3 vest:
- 6Sh46 pouch;
- 6Sh46 and VOG-25 pouches;
- 6Sh46 RPK pouch;
- SVD mags pouch;
- PM holster.

Also added 2 features:
- 6Kh4 knife is showed up depending on its presence in the inventory;
- reworked inventory structure of the vest. Now you can take items depending pockets' size. For example, in default 6B3 you can take: 4 AK mags (in stomache pouches), 4 RGD-5 (in back pouches) and 2 RPK mags (in the chest pouch).