fix: disable autofocus axis widget if no autofocus device is loaded/selected

[fdrgsp]

This PR updates the logic for when the autofocus devices (Autofocus Axis widget and Autofocus Per Position checkbox)should be disabled to then emit a correct useq.MDASequence.

On top of disabling them when an absolute z plan is selected, this PR adds:

• If no Autofocus device is loaded they are disabled (thus responding to the core systemConfigurationLoaded signal).

• They are also disabled if the core AutoFocus property is set to None (thus responding to the core propertyChnaged signal).

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.82353% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.67%. Comparing base (1cda2b7) to head (00e3a44). Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pymmcore_widgets/mda/_core_mda.py	97.77%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #386 +/- ## ========================================== - Coverage 90.71% 90.67% -0.05% ========================================== Files 85 85 Lines 9234 9290 +56 ========================================== + Hits 8377 8424 +47 - Misses 857 866 +9 ```

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