Add support for multiple button rows

[glutanimate]

This patch adds the ability to define multiple rows of buttons in the "Quick Note and Deck Buttons" Add-on. I've tried to stay as close as possible to the original implementation, but I had to monkey-patch the init methods of the two choosers to update the widget layout.

The button definition syntax is now a list of lists, where each list represents a row of buttons defined as dictionaries. If the button definitions are provided in the old syntax, the add-on automatically falls back to a single row.

Other changes included in this patch:

• Switch to new-style signals and slots (which should hopefully make the add-on future-proof for Anki 2.1)
• Handle missing note types more gracefully by notifying the user through a tooltip
• Display the current hotkey in the button tooltip if set

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Thanks again for all of your work on these add-ons! They've made my day-to-day Anki use much easier.

[ospalh]

Looks like a nice idea, thanks. I am not totally happy with this, but will pull this anyway, as it is an improvement. You can look at the next commits to see what i didn’t like. 😳

[glutanimate]

Thanks for merging this and thanks also for your thoughtful changes!