[Email] Make messageListModel sorting presistent.

Sorting will not change when account or folder keys are set, it can
be changed via the provided setSorting methods.
[Email] Add sort by read status (MessageListModel)
[Email ]Added sort by attachments and sort by priority methods.

Add hasAttachments role to messageListModel
[Email] Add sort by size and size section role.
Merge pull request #132 from VDVsx/master

[Email] Make messageListModel sorting presistent.
Merge branch 'master' of https://github.com/nemomobile/nemo-qml-plugins