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[bot]]

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 :warning:
src/napari_micromanager/_util.py	65.90%	15 Missing :warning:
src/napari_micromanager/main_window.py	71.79%	11 Missing :warning:
src/napari_micromanager/_gui_objects/_toolbar.py	75.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ 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 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.