Improve the contact invite dialog

[diederik]

As discussed here we need to update the "invite contact" dialog. The current way it works is a bit confusing.

Instead, this would be a nice way: - have two lists: one with available contacts, one with selected contacts - clicking the item should only select it (as if it's a list). - have an 'add' and 'remove' button in between (a bit like configuring toolbars)

Optionally: - an incremental search box on top - optional: drag&drop of contacts.

[sifcenter]

Implemented at r4120