[mlite] Avoid include guard conflict with mlite-global.h

Pointed out here
https://together.jolla.com/question/30505/mlite5-mlite-globalh-has-wrong-define-on-top/
[mlite] Add an MDConfGroup class.

MDConfGroup represents a directory in the heirarchy of DConf
properties. Values within that group can be read and  written using the
value() and setValue() functions.  Or QObject properties of a derived
type can be bound to keys corresponding to the property name and
automatically kept in sync.
Merge pull request #12 from adenexter/mdconfgroup

[mlite] Add an MDConfGroup class.