WIP: import contacts from remote CardDAV server

[Fnux]

This PR addresses parts of #124: it alows to import contacts from a remote CardDAV server using vdirsyncer. I did not add a way to export the local contacts since the user doesn't have a form (for now) to add new entries manually.

I still have to investigate a sexier way to store the application's configuration and write tests for the new features.

[CRImier]

Re "storing application's configuration" - we have solutions for working with .json config files - storing, recreating configs on error, saving, even primitive migrations. See the howtos regarding that, let me know if anything's unclear.

[codecov[bot]]

Codecov Report

Merging #134 into devel will increase coverage by 2.8%. The diff coverage is 37.61%.

@@            Coverage Diff            @@
##            devel     #134     +/-   ##
=========================================
+ Coverage   38.51%   41.32%   +2.8%     
=========================================
  Files         233      255     +22     
  Lines       18126    20867   +2741     
=========================================
+ Hits         6982     8623   +1641     
- Misses      11144    12244   +1100

Impacted Files	Coverage Δ
helpers/vdirsyncer.py	1.88% <1.88%> (ø)
helpers/xdg.py	100% <100%> (ø)
apps/personal/contacts/address_book.py	54.92% <33.33%> (-20.29%)	:arrow_down:
apps/personal/contacts/vcard_converter.py	33.33% <50%> (-1.15%)	:arrow_down:
apps/personal/contacts/main.py	11.49% <7.46%> (-23.93%)	:arrow_down:
apps/personal/contacts/contact.py	83.33% <83.33%> (ø)
tests/test_drivers.py	66.22% <0%> (-24.69%)	:arrow_down:
apps/example_apps/sandbox/main.py	40.9% <0%> (-22.73%)	:arrow_down:
ui/refresher.py	72.61% <0%> (-8.21%)	:arrow_down:
ui/utils.py	90.67% <0%> (-5.16%)	:arrow_down:
... and 90 more

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[Fnux]

@CRImier I'd like to move the Contact and AddressBook classes under libs/ since they might be used by other apps in the future (Calls, SMS). Does it makes sense to you?

[Fnux]

I still have to clean, document and test the code I moved to lib/. I will resolve the WIP state once it is done.

[Fnux]

I added a paths helper, which I needed for the vdirsyncer and address_book modules.

[CRImier]

This works for your usecase, right?

We can use the default config management functions to store the config files, just need to point it to the right folder.

As about testing - doctests for code in libs/ pass, but you can add more; we don't yet have a good way to test apps.

[CRImier]

Looks good! Will do a couple of fixes now and merge

[CRImier]

Oh right, read through the code again. Thinking about some kind of changes to config management that'd avoid using the main ZPUI config. For now, I'm making sure that the ZPUI config is used the least amount possible - since it has hardware configuration parameters, it's important for it to not get changed by the users often. + I'm not sure that it makes sense for the vdirsyncer wrapper library to store the config where it currently is, that is, in libs/ - the idea about libraries in libs/ is that they can be used by more than one app, in different ways.

So, I have an idea - read the config from the app (+ allow changing it from the same app), then pass the config variables. Let me know what you think of it. For now, merging the code into a separate branch (contacts-sync) + adding you as a contributor in this repo.