Make SavePage Action opt-in

The new context menu action to save the page should be made optional, and opt in.

E.g. the two qt quick browser examples do not handle it directly:

For webengine/minimal, activating 'Save' from the context menu does nothing. The second try leads to an assert:

ASSERT: "!m_ongoingDownloads.contains(info.id)" in file /home/kakoehne/dev/qt/5.7/src/qtwebengine/src/webengine/api/qquickwebengineprofile.cpp, line 174
Aborted (core dumped)

For webengine/nanobrowser a GUI is shown which seems however also disfunctional. E.g. the file is always saved in .mhtml format.

It seems the action does require some handling in the app to be functional, and therefore should be opt-in. The same goes for the widget's action.