There's nothing specific to ReadOnlyPart about the statusbar extension.
This allows to port KMailPart to KParts::Part and still have it show
items in the kontact statusbar.

Technically the overloaded ctor could be ambiguous (e.g. if called with
a nullptr argument), but such an arg wouldn't make sense, and I built
all of the git.kde.org modules that use StatusBarExtension, without
issues.