Add import/export functionality and buttons on ui

[goanpeca]

Related to napari/packaging#196

• Add support for export/import list of installed plugins
• Adds tests
• Adds documentation

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 4 lines in your changes missing coverage. Please review.

Project coverage is 94.09%. Comparing base (05728e0) to head (bd387ac).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
napari_plugin_manager/_tests/test_npe2api.py	87.50%	2 Missing ⚠️
napari_plugin_manager/base_qt_plugin_dialog.py	94.59%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #120      +/-   ##
==========================================
+ Coverage   94.07%   94.09%   +0.01%     
==========================================
  Files          13       13              
  Lines        2026     2099      +73     
==========================================
+ Hits         1906     1975      +69     
- Misses        120      124       +4     

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[psobolewskiPhD]

Sorry I missed this @goanpeca
I made a quick small followup to add tooltips: #127