QScintilla2

This adds the build file for the qscintilla2 editor component (Qt5 version).
(http://www.riverbankcomputing.com/software/qscintilla/intro)

We plan to integrate the editor component into OpenSCAD which is already using MXE for the Windows build.

Please provide feedback if there are better ways to retrieve the command line switches for building the test file.
Are you sure `qt5` is what you want? Many packages depending on qt5 only actually need `qtbase` and not all the components in src/qt5.mk. See qwt.mk for example.