Allow commercial installation on the top of open source installation

If an open source user has installed Qt and buys a commercial license, the maintenance tool does change the existing installation type. Many of our users continue commercial product development by using the open source installation accidentally. There are several problems with this:

• Customers have difficulties automatically checking the correct license use in projects. 
• Our systems show that there are a lot of unassigned licenses. 

After purchasing the license, Maintenance Tool should show a clear warning and guidance what to do:

"You have a commercial Qt license available, but this installation is open source. It is not possible to convert an open source installation to a commercial one. Please create a separate commercial installation."