[messaging] Replace MessageFolder class with plain json part 1.

[messaging] Replace MessageFolder class with plain json part 1.

+ MessageFolder::filterFolders() method converted to function and removed from the header.
+ Replace MessageUtils::folderToJson() with MessageFolder::toJSON() method.
+ Replace all MessageFolder attributes with single picojson object.
+ Move folder-related methods and types from messaging_util to message_folder module.
+ Replace jsonToFolder() method with MessageFolder(picojson::value) constructor.

[Verification] tct-messaging-*-tizen-tests 100% pass.

Signed-off-by: Michal Michalski <m.michalski2@partner.samsung.com>
Change-Id: I08143e5fa65978a6979b4a9a1753cfba8adaa3ed