Add contact dialog: reorganize input fields

To let a user enter what is most import to her or him, the dialog now
starts with the name for a new contact. Almost every contact manager
does this (take a look at email, address books (even paper address
books), IM clients, etc.) This way the user starts with what they know.

Second is the ID field which keeps the actual contact information.
Usually, a user will have a name and the address of new contact. Since
the name was entered first, (s)he will understand what has to be typed
into this field.

Third is the user's account. The group setting is last, because it is
optional.

Thanks to David Bolton for suggesting this change. This commit message
contains parts of his rational for it.
Add contact dialog: rename name and address labels

For clarity the labels for the display name and the contact's ID or
number have been renamed.

'Display name' becomes just 'Name', because 'Display Name' is
unnecessary technical terminology, and since there is no other Name
fields in the dialog, the technical distinction is unnecessary.

'ID or number' becomes 'IM address or phone number'. Refering to a phone
number is much clearer than to (any) number. The best terminology for
"ID" is less clear. However, 'IM address' is similar to 'email address'
and therefore to something users might know.

Thanks to David Bolton for suggesting this change. This commit message
contains parts of his rational for it.
Add contact dialog: focus name field by default

Usually, it is the name what a user has in mind first when adding a new
contact. Therfore and because it is the first field in the dialog, it
makes sense to focus this text field by default.
Add contact dialog: update account and group labels

The former "Select account" and "Select group" labels have been changed to just
"Account" and "Group". This fits more to to other options which say e.g. "Name"
instead of "Type name". Thanks to David Bolton or this suggestion to make the
naming more consistent.