style sheet length parsing not working as documented

The style sheet reference documentation says any length can be specified with units of pt, px, em, or ex.
https://qt-project.org/doc/qt-5.0/qtwidgets/stylesheet-reference.html#length

However the code in qcssparser.cpp differs:

• for font sizes, only pt and px work (setFontSizeFromValue())
• for other sizes, only px, em and ex work (ValueExtractor::lengthValue())