Move to App widget and call from notebook

[unkcpz]

For the test purpose of #428 proposed by @superstar54, we think moving the code from qe.ipynb into a module and creating an App class would make it not only more maintainable and easy for integrate unit test of the whole app as an widget.

[codecov]

Codecov Report

Patch coverage: 30.90% and project coverage change: -0.62% ⚠️

Comparison is base (bd7f39b) 56.52% compared to head (64232c0) 55.91%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #442      +/-   ##
==========================================
- Coverage   56.52%   55.91%   -0.62%     
==========================================
  Files          26       27       +1     
  Lines        2321     2375      +54     
==========================================
+ Hits         1312     1328      +16     
- Misses       1009     1047      +38     

Flag	Coverage Δ
python-3.10	55.91% <30.90%> (-0.62%)	⬇️
python-3.8	55.96% <30.90%> (-0.62%)	⬇️
python-3.9	55.96% <30.90%> (-0.62%)	⬇️

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Files Changed	Coverage Δ
src/aiidalab_qe/app/main.py	26.92% <26.92%> (ø)
src/aiidalab_qe/app/__init__.py	100.00% <100.00%> (ø)
src/aiidalab_qe/app/common/__init__.py	100.00% <100.00%> (ø)

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[unkcpz]

@superstar54 you mentioned the dlink may not necessary. I think it depends on the scenario and if you can decouple step widgets by adding another "data" layer in between. If you want to make such a change, please make a dedicated PR for that.

[superstar54]

LGTM!