MediaCategoryViewController: Set collectionViews contentInset when editing

(fixes #416)

Sources/MediaCategories/MediaCategoryViewController.swift

@@ -187,6 +187,7 @@ class VLCMediaCategoryViewController: UICollectionViewController, UICollectionVi
         UIView.animate(withDuration: 0.3) { [weak self] in
             self?.editToolbarConstraint?.constant = self?.isEditing == true ? 0 : VLCEditToolbar.height
             self?.view.layoutIfNeeded()
+            self?.collectionView.contentInset = UIEdgeInsets(top: 0, left: 0, bottom: self?.isEditing == true ? VLCEditToolbar.height : 0, right: 0)
         }
     }