Changes a lot

Change database and a lot of backend-functions, but nothing ready yet.
This is only a commit to store the changes.

Signed-off-by: Sven Haardiek <Sven@Haardiek.de>
Split playlist.id and playlist.order

Signed-off-by: Sven Haardiek <Sven@Haardiek.de>
Change database and endpoints

Not done yet

Signed-off-by: Sven Haardiek <Sven@Haardiek.de>
Fixed some bugs

Signed-off-by: Sven Haardiek <Sven@Haardiek.de>
Update rest-endpoints.md to document changes

Signed-off-by: Sven Haardiek <Sven@Haardiek.de>
Some minor changes

Signed-off-by: Sven Haardiek <Sven@Haardiek.de>