Update Welcome-mode contents with Qt Academy items

The contents of the Welcome-mode of Qt Creator shall be updated: 

• Replace links to existing content with Qt Academy items, if the latter are more recent.
• Remove or replace old content (some of the items date back to 2014. 

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[Evaluation and discussions about the topic|https://intranet.qt.io/display/QTPO/Qt+Creator+Welcome-mode+content+evaluation]. 

This work is only the first step along the way towards a dynamic and personalized set of items in Welcome-view, Qt Academy content can be linked to in the same way as existing Youtube-videos. Hence there is no need for code changes, only tweaking the data.