Add Account and Folder models, update CMakeLists.txt with source files, add MailItem implementation

src/core/mailitem.cpp

@@ -0,0 +1,16 @@
+#include "mailitem.h"
+
+MailItem::MailItem(qint64 id, const QString& subject, const QString& sender,
+                   const QString& recipient, const QDateTime& date,
+                   bool read, bool flagged,
+                   const QVector<QString>& attachments)
+    : m_id(id)
+    , m_subject(subject)
+    , m_sender(sender)
+    , m_recipient(recipient)
+    , m_date(date)
+    , m_read(read)
+    , m_flagged(flagged)
+    , m_attachments(attachments)
+{
+}