Add better support for weapons attachments

[marvin9257]

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From @Liko on discord -No way to add a weapons attachment (e.g. a scope) that automatically affects the weapons stats. Only DM and damage is considered currently.
Describe the solution you'd like
When dragging and dropping an attachment onto a weapon, modifiers are automatically apply to any stat.

Describe alternatives you've considered
Manual adjustment

Additional context
This improvement will need to wait until FVTT supports embedded document in items.