gallery: use 5 columns in iphone landscape

this is also what system gallery & co are doing,
also this matches the ratio of delta's ipad columns
(8/5 ~= 5/3 ~= 1.6)

maybe the old 4 columns was just a typo :)

deltachat-ios/Controller/GalleryViewController.swift

@@ -266,7 +266,7 @@ private extension GalleryViewController {
 
         // columns specification
         let phonePortrait = 3
-        let phoneLandscape = 4
+        let phoneLandscape = 5
         let padPortrait = 5
         let padLandscape = 8