Allow HTMLEditor to work with limited functionality when Qt WebKit/WebEngine is not available

Similar motivation to #1462: Qt WebKit is deprecated and its replacement WebEngine is nontrivial to build (sometimes not available).

However different from #1462, this issue is concerned about making it easy to display rich text in HTML even when WebKit/WebEngine is not available in a Qt environment. This is motivated by the observation that Qt Widgets that use QTextDocument, such as QTextEdit, are capable of displaying rich text with a subset of HTML4 markup (see https://doc.qt.io/qt-5/richtext-html-subset.html). Furthermore, the wx implementation of HTMLEditor also has limited capability in HTML rendering.

The title of this issue has assumed a specific implementation: Make this a fallback option part of HTMLEditor. However, different implementation, e.g. having a separate editor (factory) altogether may also be considered and can be discussed.