Simplified and updated code

[crud89]

Improvements:

• Largely simplified codebase compared to the original version.
• Switched implementation to use QQmlApplicationEngine and ApplicationWindow.
• Support for recent versions of OpenCascade (7.3.0) and Qt (5.12).
• Removed dependency on private headers (allows for dynamic linking).
• Build tested in Visual Studio 2019 (vc150), 2017 (vc141) and Qt Creator (4.9.2).

Removals:

• Mouse interactivity (see readme).