Replace DependentStep mixin with proper QeWizardStep MVC classes

[edan-bainglass]

This PR resolves issues with #1004 (incorrect handling of step-to-step switching/rendering) by treating the app steps more uniformly. It also introduces a simple locking mechanism on submission that disconnects all observations of step confirmation, thus rendering the app in a "locked" state by permanently disabling confirm/submit buttons.

@superstar54 note that this also simplifies the handling of "invalid" dependent steps (referencing your comment on #1004)

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 11 lines in your changes missing coverage. Please review.

Project coverage is 69.05%. Comparing base (771f408) to head (30bf188).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/aiidalab_qe/common/widgets.py	75.55%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1032      +/-   ##
==========================================
+ Coverage   69.04%   69.05%   +0.01%     
==========================================
  Files         117      117              
  Lines        6507     6506       -1     
==========================================
  Hits         4493     4493              
+ Misses       2014     2013       -1     

Flag	Coverage Δ
python-3.11	69.05% <87.50%> (+0.01%)	⬆️
python-3.9	69.08% <87.50%> (+0.02%)	⬆️

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[superstar54]

LGTM!