Moved attach selection over to the select menu.

Potentially we might want to make it a config option on an item (magazine)
	AGM_Attachable = 1;
Thus it can support new attachables easier.
Added an AGM Modifier Key.
Merge pull request #860 from CorruptedHeart/AttachSelect

Moved attach selection over to the select menu.
Merge pull request #861 from CorruptedHeart/ModifierKey

Added an AGM Modifier Key. Closes https://github.com/KoffeinFlummi/AGM/issues/850