Xbee: Code refactoring

[Shulyaka]

A little refactoring: make cluster classes non-internal for clarity and move types to a separate file.

[coveralls]

Pull Request Test Coverage Report for Build 3460741905

• 141 of 152 (92.76%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.07%) to 82.759%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
zhaquirks/xbee/init.py	47	50	94.0%
zhaquirks/xbee/types.py	86	94	91.49%

Totals
Change from base Build 3446076419:	0.07%
Covered Lines:	6336
Relevant Lines:	7656

---

💛 - Coveralls

[codecov-commenter]

Codecov Report

Base: 82.68% // Head: 82.76% // Increases project coverage by +0.08% 🎉

Coverage data is based on head (9964386) compared to base (59c098d).
Patch coverage: 92.94% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1919      +/-   ##
==========================================
+ Coverage   82.68%   82.76%   +0.08%     
==========================================
  Files         245      246       +1     
  Lines        7647     7660      +13     
==========================================
+ Hits         6323     6340      +17     
+ Misses       1324     1320       -4     

Impacted Files	Coverage Δ
zhaquirks/xbee/types.py	91.48% <91.48%> (ø)
zhaquirks/xbee/__init__.py	95.04% <94.44%> (+1.76%)	⬆️
zhaquirks/xbee/xbee3_io.py	100.00% <100.00%> (ø)
zhaquirks/xbee/xbee_io.py	100.00% <100.00%> (+33.33%)	⬆️

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[Shulyaka]

Ok, I've decided to separate the refactoring and the new functionality into two separate PRs.
This PR is for the refactoring only, no new functionality.