Transparent background not working on macOS

[snshn]

Potentially need to switch LiquidAppWindow to inherit QOpenGLWidget and then use QWebEngineView as a child for platforms that use OpenGL for rendering and not XRender (Windows, macOS). Otherwise transparency only works when Qt::FramelessWindowHint is set.

If OpenGL doesn't work, then alternatively for macOS and Windows, there could be forced frameless window with non-native window decoration (kinda like Chrome/Chromium used to have). This way transparency should work on those platforms as well.

[snshn]

Implemented and seems to be working. Has some glitches sometimes, but nothing disastrous.

[snshn]

For future references:

• https://doc.qt.io/qt-5/qtgui-openglwindow-example.html
• https://stackoverflow.com/questions/35245809/qt-opengl-translucent-objects-show-content-from-behind-window