Add ability for macros to have non-execution, non-modifier keys

[patrickberkeley]

It would be nice to be able to have macros like:

manager.addMacro({
  executionKeys: ['a', 'f'],
  type: 'keydown',
  modifierKeys: ['altKey', 'ctrlKey']
});

The above would fire when alt + ctrl + a + f are pressed simultaneously in any order.

[patrickberkeley]

@gossi we've discussed this and thought it through and are going to stick with:

0-N modifier keys + 1 execution key

As the formula for a macro. Reasons:

• The JS Web KeyEvent API is well suited for it.
• Handling multiple execution keys on the same macro would introduce a large amount of complexity in this addon's code. Here are some thing that would need to be changed: 1) keep track of all pressed execution keys, check them against registered macros directly alt + ctrl + a alt + ctrl + f, then check them against registered macros in combination with each other alt + ctrl + a + f.
• We look at desktop applications and they all follow the above formula (as far as we saw). We take this as some validation that 1) it's a solid approach, 2) users will be used to / expect this formula.