Copy and paste within QTextEdit doesn't preserve font pixel size correctly

To see the problem, build the attached demo app, select some text in the edit control and drag and drop it elsewhere within the same control. The font size should be preserved, but instead gets changed to something much smaller.

In this example the font size is being set in pixels, rather than in points. When a copy is made of text with a font size of pixels, QTextHtmlExporter doesn't correctly create a font-size attribute in the resulting HTML, which means that when it's pasted back it has a default font size.

This should be easy to fix as HTML/CSS supports pixel sizing with "px".