Add contact and contact group editor

kalendar-editing

Todos before getting this merged:

• Move collection combobox to lib/quick
• Add a few more fields to the qml (just so that it doesn't look so empty)
• better error handling (ContactEditor already emit a signal for that)
• naming, currently it's a mess with interchanging term for default/current/collection/addressbook
• handle not selected collection

For later MRs:

• add remaining fields (instant messagers, postal address, ...)
• Handle contact group
• Handle contact group in the applet
• Add nested contact group
• Make it possible to edit contact group entry (currently you can only add and delete)
• handle contact getting modified while editing (most of the code for that is already present in the backend)
• Make it possible to delete contact and move them from one collection to the other