Qt5 port of the Scintilla source code editing widget

 QScintilla is a text editor for Qt5 with features especially useful when
 writing and debugging source code. These include support for syntax
 styling, error indicators, code completion, call tips and margins.
 .
 Styling choices are more open than with many editors, allowing the use
 of proportional fonts, bold and italics, multiple foreground and background
 colours and multiple fonts.