Integrate with Gramps' APIs instead of parsing Gramps XML

[bartfeenstra]

This fixes #1147 and this fixes #696 and this fixes #417

Blocked by

• Allow Gramps' core to be imported without requiring GObject gramps-project/gramps#2016
• Improve Gramps test coverage #2490

To do

• Test on macOS again when https://gramps.discourse.group/t/gramps-api-integration/7219/28?u=bartfeenstra is addressed

To do before merge

• Set the Gramps dependency to a stable version
• Ensure GrampsApiLoader.load_xml() is marked internal
• remove the lxml, pycairo, and pygobject dependencies
• remove the newly added temporary base classes and the old loader
• Undo the changes to test.yml
• make the new loader public (not internal)

[bartfeenstra]

Remove this before merging

[codecov]

Codecov Report

Attention: Patch coverage is 98.02632% with 12 lines in your changes missing coverage. Please review.

Project coverage is 21.12%. Comparing base (6684977) to head (6928aa8).

Files with missing lines	Patch %	Lines
betty/gramps/loader/_loader.py	97.36%	9 Missing and 2 partials ⚠️
betty/gramps/loader/__init__.py	98.76%	0 Missing and 1 partial ⚠️

❗ There is a different number of reports uploaded between BASE (6684977) and HEAD (6928aa8). Click for more details.

HEAD has 14 uploads less than BASE

Flag	BASE (6684977)	HEAD (6928aa8)
	19	5

Additional details and impacted files

@@             Coverage Diff             @@
##            0.4.x    #2444       +/-   ##
===========================================
- Coverage   98.44%   21.12%   -77.32%     
===========================================
  Files         440      384       -56     
  Lines       26264    24414     -1850     
  Branches     1161     1130       -31     
===========================================
- Hits        25855     5158    -20697     
- Misses        257    19181    +18924     
+ Partials      152       75       -77     

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