Add coloration syntax

[Nodd]

Add coloration syntax to the code lines
Check that it is still readable with coloring.

[Nodd]

Turns out it's not easy to display rich text in a QTreeWidget.
I tried with QLabels (as in https://stackoverflow.com/a/17546140) but the background is wrong, see https://github.com/Nodd/lineprofilergui/tree/syntax_highlighting
Alternatives are way more involved: https://stackoverflow.com/questions/1956542/how-to-make-item-view-render-rich-html-text-in-qt