[milestone 2 & 3] (WIP) Contact parsing

[kxxt]

TODOs:

• [x] Add a virtual address book that represents all contacts
• [ ] Performance optimization: cache the categories and sync them with thunderbird in background.js
• Calc the max width of category tree in js. (Not sure if this is truly necessory. I found 15rem works well in most cases. (I do not think it worth doing now)
• [ ] There is a subtle bug: when the popup opens, it displays on the right side first but shifts to the left side after loading. (not sure how to fix it)

@jobisoft Please add more TODOs here if I forget something.

[kxxt]

This PR is ready for review if we postpone the implementation of cache.

There is a subtle bug I can't fix: when the popup opens, it displays on the right side first but shifts to the left side after loading. (not sure how to fix it)