Addressbook plugin improvements

Hi,

I have been looking into many LDAP based address book programs to find one 
that can replace our database base solution. So far find Luma to have the 
best balance between flexibility and user-friendliness.

While playing around with it, I stumbled upon two bugs, for which I have added 
a fix in the attached diff.

1. The addressBox is not properly reset when switching contacts.
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To reproduce:
* Select a contact in the addressbook plugin
* Click on the Phone/Address tab
* Select "Other" in the addressBox
* Now select another contact
=> The addressBox will continue to display "Other" while the address actually 
being displayed is the "Business" address

Fix: reset the addressBox to "Business" when changing contacts

2. Addresses are not submitted to LDAP in an RFC 2252 compliant way
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RFC 2252 requires a specific syntax for attributes of type "Postal Address" 
(freely translated):
- newlines should be replaced with $-signs
- existing $-signs in the address should be encoded in hex notation ('\24')
- existing \-characters in the address should equally be encoded (as '\5c' 
or '\5C')
Luma currently doesn't do this.