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/debuggeritem.cpp

@@ -95,7 +95,7 @@ DebuggerItem::DebuggerItem(const QVariantMap &data)
     m_lastModified = data.value(QLatin1String(DEBUGGER_INFORMATION_LASTMODIFIED)).toDateTime();
 
     foreach (const QString &a, data.value(QLatin1String(DEBUGGER_INFORMATION_ABIS)).toStringList()) {
-        Abi abi(a);
+        Abi abi = Abi::fromString(a);
         if (!abi.isNull())
             m_abis.append(abi);
     }