Web-based core update in the ACP

[fiammybe]

Dependent on #1636 and #1637
This would be one for power users: offer a button that lets you upgrade the ImpressCMS core from the version page in the ACP.

• Download the latest version
• verify signature of the downloaded file
• Backup your current site
• unzip the file --> improvement of the Icms core might be necessary
• run upgrade script (if available)
• If something goes wrong : offer the option to restore the latest backup