Shortcuts: Allow configuring keybinds for launching dock items

[leolost2605]

Goes with elementary/dock#245

They show up but didn't test because of #512

Also the phrasing might need some improvement :)

[zeebok]

Is this still a valid PR to review? The blog says the dock now has the keyboard shortcuts.

Edit: Oh this is to change the shortcuts to launch items 0-9? Makes sense.

[zeebok]

I think the language is okay for an initial version. I certainly can't think of something better off of the top of my head.

[danirabbit]

I'd still like to see a smarter way to set all of these at once in the future, but yeah this is good as a first version :)