Mail: Add an e-mail application called Mail

This utilises LibIMAP and LibWeb to provide an e-mail client.

The only way currently to connect to a server and login is with a
config file. This config file should be stored in ~/.config/Mail.ini
Here is an example config file:
```
[Connection]
Server=email.example.com
Port=993
TLS=true

[User]
Username=test@example.com
Password=Example!1
```

Since this is stored in plaintext and uses a less secure login method,
I'd recommend not using this on your main accounts :^)

This has been tested on Gmail and Outlook. For Gmail, you either have
to generate an app password if you have 2FA enabled, or enable access
from less secure apps in your account settings.

Userland/Applications/Mail/InboxModel.cpp

@@ -0,0 +1,50 @@
+/*
+ * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "InboxModel.h"
+
+InboxModel::InboxModel(Vector<InboxEntry> entries)
+    : m_entries(move(entries))
+{
+}
+
+InboxModel::~InboxModel()
+{
+}
+
+int InboxModel::row_count(GUI::ModelIndex const&) const
+{
+    return m_entries.size();
+}
+
+String InboxModel::column_name(int column_index) const
+{
+    switch (column_index) {
+    case Column::From:
+        return "From";
+    case Subject:
+        return "Subject";
+    default:
+        VERIFY_NOT_REACHED();
+    }
+}
+
+GUI::Variant InboxModel::data(GUI::ModelIndex const& index, GUI::ModelRole role) const
+{
+    auto& value = m_entries[index.row()];
+    if (role == GUI::ModelRole::Display) {
+        if (index.column() == Column::From)
+            return value.from;
+        if (index.column() == Column::Subject)
+            return value.subject;
+    }
+    return {};
+}
+
+void InboxModel::update()
+{
+    did_update();
+}