fix: connect core roiSet signal to MDAWidget _core_grid and _core_position

[fdrgsp]

Currently, when the camera ROI is changed, any previously added grid_plan or positions containing a grid_plan subsequence do not automatically update their field of view size.

This PR fixes this bug mentioned in #405 .

closes #405

[codecov]

Codecov Report

Attention: Patch coverage is 86.36364% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.64%. Comparing base (43fd386) to head (817de7a).

Files with missing lines	Patch %	Lines
src/pymmcore_widgets/mda/_core_positions.py	85.71%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #406      +/-   ##
==========================================
- Coverage   90.65%   90.64%   -0.02%     
==========================================
  Files          85       85              
  Lines        9362     9383      +21     
==========================================
+ Hits         8487     8505      +18     
- Misses        875      878       +3     

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