feat: restore the last napari-micromanager layout at startup

[fdrgsp]

with this PR, when we save the state of the QMainWindow in a json file so that when we re-open napari-micromanager again the saved layout is restored.

the json file has two keys:

• layout_state: where the state of the napari QMainWindow is stored using the saveState() method (the state is base64 encoded to be able to save it to the json file)
• pymmcore_widgets where the names of the docked pymmcore_widgets are stored. The "pymmcore_widgets" key is crucial in our layout saving process. It stores the names of all active pymmcore_widgets at the time of saving. Before restoring the layout, we must recreate these widgets. If not, they won't be included in the restored layout (this is because the pymmcore-widgets are not created at the startup but only when the user clicks on their QPushButton ).

The json file is saved in the appdirs.user_data_dir path in a folder named napari_micromanager.

This PR also adds a ScrollableWidget to house the pymmcore_widgets. This allows for easy docking of widgets either above or below others, ensuring they remain within the monitor's display area and do not overflow off-screen.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.00000% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 79.23%. Comparing base (4c625fa) to head (bb95a68).

Files	Patch %	Lines
src/napari_micromanager/_gui_objects/_toolbar.py	89.83%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #317 +/- ## ========================================== + Coverage 78.24% 79.23% +0.99% ========================================== Files 13 13 Lines 625 684 +59 ========================================== + Hits 489 542 +53 - Misses 136 142 +6 ```

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