apply add_widget also to toolbar specified using str (missing in #117)

[seb5g]

No description provided.

[seb5g]

@Ashwolaa your PR #117 was missing the toolbar mechanism for add_widget. This solves this

[Ashwolaa]

nice catch

[codecov]

Codecov Report

❌ Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.24%. Comparing base (b25a9b4) to head (075e0b0).
⚠️ Report is 2 commits behind head on 5.0.x_dev.

Files with missing lines	Patch %	Lines
src/pymodaq_gui/managers/action_manager.py	44.44%	5 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           5.0.x_dev     #120      +/-   ##
=============================================
- Coverage      64.26%   64.24%   -0.03%     
=============================================
  Files             73       73              
  Lines           9132     9137       +5     
=============================================
+ Hits            5869     5870       +1     
- Misses          3263     3267       +4     

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