Randomize additional vendors

[Dinopony]

Add options to randomize specific vendors, namely:

• armor vendors at Dobbo, Boldan, Snivelak and Grelbin (putting armor upgrades in the item pool)
• slim's mod vendors at Endako, Tabora and Boldan (putting weapon mods in the item pool)
• slim's shack vendor (putting spaceship upgrades in the item pool)

The weapon mods vendor needs to be implemented carefully since players can lock themselves out of good items by spending all their platinum bolts on bad items.
Several solutions can be imagined for that:

• turning those locations into regular bolt purchases instead of platinum bolts (turning plat bolts into outright useless items, that could be put back into use through a potential revamp of the "Cheats" menu into a "Rewards" menu for locations unlocked on some platinum bolt thresholds instead of cheats unlocked on some skill point thresholds)
• make the vendor slots sequential (you are forced to buy them in order, preventing any possibility for a lockout)