Abi: Clean up constructors

Only have one constructor. Get rid of implicit conversion from QString
to Abi -- that seems rather non-obvious. Use Abi::fromString(...) instead.

Change-Id: Ic638d3d4022c465123449089b0679197a5eb445d
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/debugger/debuggeritemmanager.cpp

@@ -356,7 +356,7 @@ DebuggerItem DebuggerItemConfigWidget::item() const
     foreach (const QString &a, m_abis->text().split(QRegExp(QLatin1String("[^A-Za-z0-9-_]+")))) {
         if (a.isNull())
             continue;
-        abiList << a;
+        abiList << Abi::fromString(a);
     }
     item.setAbis(abiList);
     item.setVersion(m_versionLabel->text());