key modifiers to M1/M2 powers

[stefanbeller]

Migrated from https://github.com/clintbellanger/flare/issues/412, originally opened by @aragan

Idea is rather simple, when you run out of mana using keyboard to activate power with pointing with mouse is quite inefficient, holding shift could switch to second set of M1/M2 assigned powers, assigned in same way that is, dragged while shift is pressed.

@clintbellanger mentioned: Something like this could work. Note that shift currently has a use for mouse_move mode, to attack without moving.

Also note, I'd like to add default powers that are used when running out of mana. E.g. if you try to use Multi-Shot but have zero mana, you will automatically use regular Shoot.