WelcomPage: Fix intitial filtering by Qt version

When having multiple Qt versions with examples registered in Creator,
the welcome mode showed all of them initially.

The reason was that m_uniqueQtId was initialized with noQtVersionsId
instead of uniqueQtVersionIdSetting().

The function uniqueQtVersionIdSetting() gets the correct Qt version from
the settings as shown in the combobox.

Task-number: QTCREATORBUG-10790

Change-Id: I9f435f6372f8b0c0495daf6c6efcf4bcc5e70455
Reviewed-by: Eike Ziller <eike.ziller@digia.com>