fix: Only allow YAML save/load when YAML available

[gselzer]

This PR avoids exceptions arising from the attempt to save/load MDA settings from YAML files, when pyyaml is not installed.

Notably, I made the decision to prevent the selection of a YAML file within the QFileDialog. We could have alternatively shown it, but prevented the exception (through a try/except or other means) instead. I prefer this method as it leaves no room for unexpected behavior (namely a user trying to save into a YAML file and then save not being completed).

Closes #333

[tlambert03]

very nice

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.59%. Comparing base (7ec66e5) to head (eaaf226).
Report is 23 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pymmcore_widgets/useq_widgets/_mda_sequence.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #347      +/-   ##
==========================================
- Coverage   90.59%   90.59%   -0.01%     
==========================================
  Files          74       74              
  Lines        8379     8385       +6     
==========================================
+ Hits         7591     7596       +5     
- Misses        788      789       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[gselzer]

Thanks codecov!

[tlambert03]

lol