Fix QToolBarExtension icon overlapping default icon

[5yutan5]

Current stylesheet overrides QToolBar extension icon.
But the default extension icon is showed when changed QToolBar orientation.
In this case, default extension icon and override icon overlap.
For fix this problem, I only show extension icon when QToolBar orientation is horizontal.

setup_style uses QProxyStyle to override the default icon, so we can override the default icon even when the orientation changed.

[codecov]

Codecov Report

Merging #205 (c1a36c8) into main (ddc345e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #205   +/-   ##
=======================================
  Coverage   98.35%   98.35%           
=======================================
  Files          39       39           
  Lines        1399     1399           
=======================================
  Hits         1376     1376           
  Misses         23       23           

Impacted Files	Coverage Δ
qdarktheme/_resources/_template_stylesheet.py	100.00% <ø> (ø)

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