[FIX] WebEngineView: Insert JS if loading already started

[nikicc]

Issue

When loading schemas with widgets that extend Orange.widgets.highcharts.Highchart Orange sometimes hangs with JS error:

js: Uncaught ReferenceError: fixupPythonObject is not defined

Description of changes

WebEngineView._onloadJS also calls self.page().scripts().insert(script) and not just connects it to self.loadStarted. In case that loading has already started (e.g. if WebEngineView receives QUrl among parameters to init) connecting of self.loadStarted occurs too late.

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #2230 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2230      +/-   ##
==========================================
+ Coverage   72.17%   72.17%   +<.01%     
==========================================
  Files         318      318              
  Lines       54773    54774       +1     
==========================================
+ Hits        39533    39534       +1     
  Misses      15240    15240

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