Externalize merc inventory

[JAsmine-ja2 NA]

Since Proedit can not really handle the items when the newly externalizations are used, it is vital to externalize the merc inventory, if we want to make items fully moddable via json.

Example: If you create a magazine in an item index that was previously empty, you won't be able to edit the quantity of this new magazine via proedit, since proedit still assumes, that

"ubPerPocket": 0 .

So, in order to change the quantity of this new magazine, an externalized merc inventory would be the best solution.

[JAsmine-ja2 NA]

Since issue #154 was solved, this is in my opinion the most pressing issue at the moment, when it comes to making the guns moddable.