Mail: Fetch and display date in e-mail list

Added a third column to show date of e-mail, as sent from the IMAP
server.

Userland/Applications/Mail/InboxModel.h

@@ -13,6 +13,7 @@
 struct InboxEntry {
     DeprecatedString from;
     DeprecatedString subject;
+    DeprecatedString date;
 };
 
 class InboxModel final : public GUI::Model {
@@ -20,6 +21,7 @@ public:
     enum Column {
         From,
         Subject,
+        Date,
         __Count
     };