Add support for enabling selectable text

Static text (text labels and similar) is currently not selectable (for copying to the clipboard) by default in Qt. This matches common desktop application behavior.

For Qt on WebAssembly the expectations are different - on the web platform users expect that text is generally selectable.

We should consider adding a way for the platform to provide the behavior (QTBUG-116044).

UI controls which already supports text selection are in-scope. On the Qt Widgets side this would be widgets which support Qt::TextInteractionFlags. On the Qt Quick side we would should consider QTBUG-14077 again, if we decide to go ahead with this feature.

There might also be a use-case for developers overriding the default. This is tracked in QTBUG-116046.