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/core/Entry.h

@@ -107,6 +107,7 @@ public:
     QString attribute(const QString& key) const;
     QString totp() const;
     QSharedPointer<Totp::Settings> totpSettings() const;
+    int size() const;
 
     bool hasTotp() const;
     bool isExpired() const;