Several things emerged from my brief trawl through examples and help today:

• The default text editing component is rudimentary, but a WebKit browser control can be used to provide sophisticated editing.
• QT has a similar system of Actions associated with widgets as Delphi does.
• The QT QIcon object seems to handle SVG, and will handle scaling automatically if necessary (at least, scaling downwards), as well as the generation of disabled variants. This promises to make action icons simpler than they are in Delphi.