Allow controllers to configure which buttons are modifiers

[A7x49]

The number of shortcuts available to controllers is very limited, but modifier buttons allow a significant increase.

Xbox controllers have a total of 14 non-modifier buttons:
- (4) X, Y, A, B
- (4) Directional buttons
- (2) menu buttons
- (2) RB, LB
- (2) analog stick buttons

There are 4 possible modifiers:
- no modifiers
- RT
- LT
- RT+LT

This gives a total of 14*4=56 possible controller shortcuts. This is not enough, especially when adding mods.

I think the number of modifier states is actually 2^M where M is the number of modifier buttons if we assume that any combination of modifier buttons creates a unique state. With 2 modifiers (RT+LT) 2^2 = 4. With 3 modifiers we would have 8 possible modifier states.

So say we had the option to add RB and LB as modifiers instead of regular buttons. We would now only have 12 regular buttons, but 4 modifiers. 4 Modifiers means 2^4=16 modifier states. 16 * 12 is 192 possible shortcuts.

I am requesting this since I am running out of possible shortcuts with my controller. I am not sure I would chose RB and LB as my extra modifiers, I would more likey use directional buttons. I'd request that users can chose themselves what buttons are modifiers, as they are used to being able to do when defining Steam Controller configurations.