Add "Size" column

- Sizes are displayed in B, KiB, MiB, and GiB with 2 significant
  digits after the decimal
- Column is hidden by default
- TestEntryModel updated for testing proxy model

src/gui/entry/EntryModel.h

@@ -43,7 +43,8 @@ public:
         Accessed = 9,
         Paperclip = 10,
         Attachments = 11,
-        Totp = 12
+        Totp = 12,
+        Size = 13
     };
 
     explicit EntryModel(QObject* parent = nullptr);