Support migrating merged GConf trees.

Check for and parse a ~/.gconf/%gconf-tree.xml file during migration.
This is mainly for Debian, which I think accidentally merged some users'
GConf trees at some point.

This also adds a command-line program (evolution-scan-gconf-tree-xml),
which parses an arbitrary %gconf-tree.xml file as a recovery measure.

(cherry picked from commit fd3ad46d19a02c4ea73117c9ebee7913ca388890)