Inventory window optimisation

[AKryukov92]

Actual for version 75c9240
As far as I understand, function crafting.make_result_selector is called on each update of player inventory: on opening, on filtering and on switching pages.
It does a lot of string concatenation, which can be avoided.

I propose to store inventory formspecs in special array in crafting object for each filtering query and for each inventory page.
It will take more server memory at the cost of speed of construction inventory screen window