composer-ng: Use QString::toDouble() in fixHtmlFontSize; add unit tests

Use QString::toDouble() in fixHtmlFontSize, as actual font sizes could well be
double.

Add unit tests for fixHtmlFontSize and toCleanHtml helper methods, that
should help with porting QRegExp to QRegularExpression.
1 job for !5 with work/ahmad/prep-qre-autotest in 28 minutes and 43 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Build
passed #61647
linux kf5-qt5 qt5.12

00:28:43

35.0%