Index sync: explicitly select contacts in address book

[enkore]

When syncing an address book it should be possible to manually (de)select each contact for the index sync.

[thechauffeur]

@audax said this is more strait forward than it seems. Just add an UI element between reading the address book and syncing while also keeping the list (which contact was not unselected) saved locally on the device.

The default should be all contacts are selected and the user may choose to unselect (and select) a few with a button to unselect all contacts.