Update dropdowns on value changes (not index)

[CasperWA]

Fixes #357

Avoid using the private method _get_grouping_label_value() for ipywidgets_extended.DropdownExtended, and use the label and value attributes instead.

Note: The latest version (v1.1.1) of ipywidgets_extended should be the minimum requirement prior to this being merged.
Edit: With the merge of #359 this is now the case.

[codecov]

Codecov Report

Merging #358 (ffeae6e) into develop (68e2df5) will decrease coverage by 0.01%.
The diff coverage is 16.66%.

@@             Coverage Diff             @@
##           develop     #358      +/-   ##
===========================================
- Coverage    35.26%   35.24%   -0.02%     
===========================================
  Files           19       19              
  Lines         2402     2403       +1     
===========================================
  Hits           847      847              
- Misses        1555     1556       +1     

Flag	Coverage Δ
optimade-client	35.24% <16.66%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
optimade_client/subwidgets/provider_database.py	15.82% <16.66%> (-0.06%)	⬇️

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