target,sdk: Link to the multimedia demopack location

In the past our Tracker package used to ship a downstream patch with a GSetting override instructing it to look into /home/shared as well to search for files to be indexed, as that's where we install the contents of the multimedia demopack.

We're no longer carrying that downstream patch, so do a quick workaround to make the multimedia files available by making the ~/Music, ~/Pictures and ~/Videos be symlinks pointing to the folders in /home/shared with the same names.

This has the downside that switching locale or the XDG dir configuration may make the multimedia files unindexed and that installing new multimedia files requires that the /home/shared subfolders are writable by the user.