Allow to change position for [number] in filename

[ackvf]

Using the option Add number to filename when saving only allows to prepend the whole filename with the number.
This is unfortunate when later I want to make selections from multiple directories, where the prepended number causes change of order.

I propose to allow to control the position of the number using [number] placeholder.

Moreover, this number seems to be unique per folder. My next proposal is to add another number that counts all generations globally.
Let's say [number_global].