message: update implementation

Add more of the Message class (and various helpers), which are to replace all
the `mu-msg-*` code.

Add more tests.

lib/message/mu-message-part.cc

@@ -0,0 +1,85 @@
+/*
+** Copyright (C) 2022 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+**
+** This program is free software; you can redistribute it and/or modify it
+** under the terms of the GNU General Public License as published by the
+** Free Software Foundation; either version 3, or (at your option) any
+** later version.
+**
+** This program is distributed in the hope that it will be useful,
+** but WITHOUT ANY WARRANTY; without even the implied warranty of
+** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+** GNU General Public License for more details.
+**
+** You should have received a copy of the GNU General Public License
+** along with this program; if not, write to the Free Software Foundation,
+** Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+**
+*/
+
+
+#include "mu-message-part.hh"
+#include "mu-mime-object.hh"
+#include "mu-utils.hh"
+
+using namespace Mu;
+
+MessagePart::MessagePart(const Mu::MimeObject& obj):
+	mime_obj{std::make_unique<Mu::MimeObject>(obj)}
+{}
+
+MessagePart::MessagePart(const MessagePart& other):
+	MessagePart(*other.mime_obj)
+{}
+
+MessagePart::~MessagePart() = default;
+
+
+Option<std::string>
+MessagePart::filename() const noexcept
+{
+	if (!mime_obj->is_part())
+		return Nothing;
+	else
+		return MimePart(*mime_obj).filename();
+}
+
+Option<std::string>
+MessagePart::mime_type() const noexcept
+{
+	if (const auto ctype{mime_obj->content_type()}; ctype)
+		return ctype->media_type() + "/" + ctype->media_subtype();
+	else
+		return Nothing;
+}
+
+size_t
+MessagePart::size() const noexcept
+{
+	if (!mime_obj->is_part())
+		return 0;
+	else
+		return MimePart(*mime_obj).size();
+}
+
+
+Option<std::string>
+MessagePart::to_string() const noexcept
+{
+	if (mime_obj->is_part())
+		return MimePart(*mime_obj).to_string();
+	else
+		return mime_obj->object_to_string();
+}
+
+
+
+Result<size_t>
+MessagePart::to_file(const std::string& path, bool overwrite) const noexcept
+{
+	if (!mime_obj->is_part())
+		return Err(Error::Code::InvalidArgument,
+			   "not a part");
+	else
+		return MimePart(*mime_obj).to_file(path, overwrite);
+}