How to convert a Windows Server 2012 Core to a Windows Server 2012 GUI installation

From the command line we start powershell:
start powershell
Optionally we set a system proxy if we don't have a direct internet connection:
netsh winhttp set proxy proxy.company.com:3128
Then we enter this command so that the actual conversion take place:
Install-WindowsFeature server-gui-mgmt-infra,server-gui-shell
If we can't access the install files from the Internet  then we can use the installation dvd:
Insert installation dvd to the drive and enter the command
Get-WindowsImage -ImagePath D:\Sources\Install.wim
where D: is the dvd drive letter
From there we find the index number of the windows server installation image we have. Using this information we can enter the command
Install-WindowsFeature server-gui-mgmt-infra,server-gui-shell -Source wim:d:\sources\install.wim:2
where 2 is the index number of the installation image we have on our server.
Finally we must restart our server using the powershell command
Restart-Computer
After the restart our server becomes a full gui installation.
To reset the proxy server settings we must enter the command
netsh winhttp reset proxy