Bug 622414 - Port magnifier to GSettings

Bug 622414 - Port magnifier to GSettings

The Shell is the only user of the magnifier, so there's no
reason to keep using GConf for its settings. Since it's a
session-wide tool, use a distinct schema,
org.gnome.accessibility.magnifier, stored in
/desktop/gnome/accessibility/magnifier just like before.
Put these settings in a separate schema file for clarity.

Old enums in GConf were stored as integers, we now use the
facilities provided by GSettings to save them as strings,
and convert them to integers automatically thanks to the
mapping stored in the schemas. Remove hard-coded default values,
which we can get from the schemas.

https://bugzilla.gnome.org/show_bug.cgi?id=622414