TYPO3 CMS logo has no link and needs to be customized

• Add a configuration, that the link always leads to the root page of the current site
• Add a configuration, that the logo of a site can be exchanged with a site specific one. It is planned, that sites can have their customized logos with a text sublime (f.e. Extensions or get.typo3). Aim is, that users can identify, that they are at a subsite.