Make integration tests work with both PHP 7.3 and 7.4

The "Trashbin" and "WebDav" traits were using each other in a circular
dependency ("WebDav" -> "Sharing" -> "Provisioning" -> "BasicStructure"
-> "Trashbin" -> "WebDav"). In PHP 7.3 this worked fine, but in PHP 7.4
the fatal error "Trait 'WebDav' not found in .../Trashbin.php" was
thrown. To solve this now the "TrashBin" trait no longer explicitly uses
"WebDav".

However, due to this change, the class using "TrashBin" is now expected
to also use "WebDav". As the "Trashbin" trait was not needed by most
contexts using the "BasicStructure" trait "Trashbin" was removed from it
and added only to those contexts that actually need it.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>