[nemo-qml-plugin-email] Add a getter to attachment sizes.

[dcaliste]

This property should be used in a for loop in EmailComposer.qml.

When re-editing a draft with an attachment, opening the attachment page raises a warning for a non existent fileSize property in the model. This is due to the fact that the model in this QML page is not an Email.AttachmentListModel but a plain QML ListModel, populated in the EmailComposer.qml page from an Email.AttachmentListModel, while the size property of the latter is not propagated to the former (under the name fileSize).

[pvuorela]

Makes sense, already from consistency point of view as most of the roles as available via specific getters.

Now checking that list I noticed the last "Index" role which feels strange (index should be available without a role etc), but it doesn't either have handling in the ::data() method. Could remove? Looks like it was there on initial commit of the model.

[dcaliste]

Thanks for the review, I've added a commit removing the Index role. Indeed, I've no idea what it can be used for. In case there is a use-case, it can still be readded.