This commit changes the new configuration system to use monitors.xml
instead of monitors-experimental.xml. When starting up and the
monitors.xml file is loaded, if a legacy monitors.xml file is
discovered (it has the version number 1), an attempt is made to migrate
the stored configuration onto the new system.

This is done in two steps:

1) Parsing and translation of the old configuration. This works by
parsing file using the mostly the old parser, but then translating the
resulting configuration structs into the new configuration system. As
the legacy configuration system doesn't carry over some state (such as
tiling and scale used), some things are not available. For tiling, the
migration paths makes an attempt to discover tiled monitors by
comparing EDID data, and guessing what the main tile is. Determination
of the scale of a migrated configuration is postponed until the
configuration is actually applied. This works by flagging the
configuration as 'migrated'.

2) Finishing the migration when applying. When a configuration with the
'migrated' flag is retrieved from the configuration store, the final
step of the migration is taken place. This involves calculating the
preferred scale given the mode configured, while making sure this
doesn't result in any overlapping logical monitor regions etc.

https://bugzilla.gnome.org/show_bug.cgi?id=777732