[WIP/RFC] Report incremental updates with listener_add()

[bstaletic]

PR Prelude

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• [x] I have read and understood YCM's CONTRIBUTING document.
• [x] I have read and understood YCM's CODE_OF_CONDUCT document.
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Why this change is necessary and useful

This is the client side for ycm-core/ycmd#1480

Missing features:

• [x] Don't leak vim hacks into ycmd.
• [x] Find out why are the first two updates full, before switching to incremental updates.
• [x] Undo and redo shouldn't lag behind a change.

[Please explain in detail why the changes in this PR are needed.]

---

This change is 

[codecov[bot]]

Codecov Report

Merging #3766 (fc3e84e) into master (9ee19df) will decrease coverage by 12.46%. The diff coverage is 92.50%.

@@             Coverage Diff             @@
##           master    #3766       +/-   ##
===========================================
- Coverage   91.29%   78.82%   -12.47%     
===========================================
  Files          27       27               
  Lines        3711     3741       +30     
===========================================
- Hits         3388     2949      -439     
- Misses        323      792      +469     

[bstaletic]

Updated to work with ycm-core/ycmd#1506

[puremourning]

Seems abandoned. We can always pick this up again in future.