[nemo-qml-plugin-email] 'First char' properties added to EmailMessageListModel

These properties can be used as section header properties in list views.
[nemo-qml-plugin-email] Using MLocale::indexBucket for extracting the first char.
Merge pull request #53 from kananoja/section_header_properties

[nemo-qml-plugin-email] 'First char' properties added to EmailMessageListModel