Add back dialogs

[altendky]

Draft for:

• Dialogs shouldn't be private...
• Documentation
• Generalize (or remove for now) the retry mechanism
  • https://tenacity.readthedocs.io/en/stable/ or such

[codecov]

Codecov Report

Merging #2 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master        #2    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           25        27     +2     
  Lines         1025      1416   +391     
  Branches        56        76    +20     
==========================================
+ Hits          1025      1416   +391     

Impacted Files	Coverage Δ
qtrio/__init__.py	100.00% <100.00%> (ø)
qtrio/_exceptions.py	100.00% <100.00%> (ø)
qtrio/_qt.py	100.00% <100.00%> (ø)
qtrio/_tests/test_dialogs.py	100.00% <100.00%> (ø)
qtrio/_tests/test_qt.py	100.00% <100.00%> (ø)
qtrio/dialogs.py	100.00% <100.00%> (ø)

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