Avoid duplicate use of overridden standard icons in setup_theme()

[5yutan5]

qdarktheme.setup_theme override Qt standard icons by:

• Override QStyle.standardIcons
• Set stylesheet icon definitions

There is no need to use stylesheet icon definitions on qdarktheme.setup_theme. Separate QSS's definition that override Qt standard icons, and avoid to override standard icons in stylesheet when qdarktheme.setup_theme is used.

This change also improves overall performance.

[codecov[bot]]

Codecov Report

Merging #213 (8011071) into main (ed6310b) will increase coverage by 0.08%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #213      +/-   ##
==========================================
+ Coverage   98.31%   98.39%   +0.08%     
==========================================
  Files          39       39              
  Lines        1422     1432      +10     
==========================================
+ Hits         1398     1409      +11     
+ Misses         24       23       -1     

Impacted Files	Coverage Δ
qdarktheme/_main.py	100.00% <100.00%> (+1.78%)	:arrow_up:
qdarktheme/_resources/__init__.py	100.00% <100.00%> (ø)
qdarktheme/_resources/_template_stylesheet.py	100.00% <100.00%> (ø)
qdarktheme/_style_loader.py	100.00% <100.00%> (ø)

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