Design improvement

[FabriceSalvaire]

• Profiling: where are the hotspots (found pygments lexer issue, pygit2 warmup ???)
• Is PyQt5 really suitable ?
• Reimplement using QML ?
• Is there a better diff viewer widget implementation ? Using QML ? Using QWebEngine ?

Actual design:

• use http://doc.qt.io/qt-5/qtextbrowser.html to render the text and to get box size
• use QSplitterHandle
• overload painEvent on both QTextBrowser and QSplitterHandle to draw link lines