[nemo-qml-plugin-email] Fix bug in isAccountValid() and isMessageValid()

Check if account and message actually exist in the DB.
Merge pull request #22 from kananoja/master

[nemo-qml-plugin-email] Some more QVariants changed to basic types
Merge pull request #21 from VDVsx/master

Fix bug in isAccountValid() and isMessageValid()
[nemo-qml-plugin-email] Register EmailFolder as a qml type