feat: dialog to select config at startup + store configs paths

[fdrgsp]

This is a first attempt to add a startup widget to select a system configuration when napari-micromanager is loaded or to create a new one through the ConfigWizard widge.

In addition to a simple widget with a combo at startup, this PR also adds a way to store the configs path in a json file (in the user directory) to keep track of them every time a user opens napari-micromanager.

This PR also adds a menu for now only containing system configuration related stuff (save and load) including the ConfigWizard widget to create new configurations.

[codecov]

Codecov Report

Attention: Patch coverage is 77.50000% with 45 lines in your changes missing coverage. Please review.

Project coverage is 77.70%. Comparing base (4c625fa) to head (12802d4).
Report is 4 commits behind head on main.

Files	Patch %	Lines
src/napari_micromanager/_init_system_configs.py	84.40%	17 Missing ⚠️
src/napari_micromanager/_util.py	65.90%	15 Missing ⚠️
src/napari_micromanager/main_window.py	71.79%	11 Missing ⚠️
src/napari_micromanager/_gui_objects/_toolbar.py	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #323      +/-   ##
==========================================
- Coverage   78.24%   77.70%   -0.54%     
==========================================
  Files          13       14       +1     
  Lines         625      812     +187     
==========================================
+ Hits          489      631     +142     
- Misses        136      181      +45     

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