Draft: New plugin: LDAP Search

Add a new plugin that resolves contact names via LDAP queries.

Normally, in AD environments, XMPP servers are integrated with LDAP such that contact information (VCard / Roster) is queried from the LDAP server and sent to the client via XMPP.

When using Gajim with Cisco's Jabber server (CUCM), I discovered that Cisco integrates LDAP by making the client do the LDAP queries itself, without the server knowing anything about it.

This plugin adds nickname resolution via LDAP, and user search (with a customizable LDAP query) to the "New Chat" dialog to match Cisco's client.

I am currently using this plugin daily and it works great, but I would like to do some more testing before merging :)