Transparent Rounded Corner of Widgets

For customized widgets (derived from QWidget, QDialog, for example), we may want to set border radius and make rounded corner transparent. We can achieve this, by:

1. Override paintEvent(...), add Qt style sheet support to customized widgets.
2. Set translucent attribute by: setAttribute(Qt::WA_TranslucentBackground);. Maybe we also need to set Qt::FramelessWindowHint window flags for dialogs.
3. Set border radius of the widget, using border-radius style sheet.

Then, we get a round-corner widget.