Make Qt WebEngine/WebKit an optional dependency for traitsui.api

[kitchoi]

This issue is for making Qt WebEngine/WebKit an optional dependency of traitsui.api such that this module can still be used as the convenient entry point when (1) HTMLEditor is not needed (2) Qt is required but Qt WebEngine/WebKit are not available.

More specifically, this can be done by making HTMLEditor an optional feature of traitsui.api conditional on whether Qt is used and WebEngine/WebKit is available.

Motivation:

• Qt WebKit has been deprecated in favour of WebEngine. Building (and importing) Qt WebEngine is a nontrivial business (currently PySide2 from EDM comes with neither). HTMLEditor is not as commonly required a feature.

Context:

• traitsui.api is provided for downstream projects to import various features of TraitsUI. This issue is only about the API module and keeping it convenient. Whether importing traitsui.editors.html_editor should fail when the web dependencies are missing is an orthogonal matter.

Related:

• ArrayEditor requires NumPy and is also an optional feature of traitsui.api. Currently a warning is emitted but it does seem noisy (see https://github.com/enthought/traitsui/issues/731)
• PythonEditor and PythonShell are optional features of pyface.api as well, see https://github.com/enthought/pyface/pull/796 (and its referenced issues)

[kitchoi]

Note that there exists traitsui.editors.api which contains a subset of traitsui.api. This issue probably applies to traitsui.editors.api as well. That said, offline discussioin suggests that traitsui.api is recommended over traitsui.editors.api.