Glib 2.42 fixes : Parameters are not editable

I was "force" to add a preprocessor directive due to glib changes...

Saying that, now we must build and ship for OS coming with glib >= 2.42
Fix glib version condition
Fix #715: Glib 2.42 issue for keyframes list. Thanks to Ron Bentley.

See http://www.synfig.org/issues/thebuggenie/synfig/issues/715
Always use "By Layer Default" blend method on startup.

This allows to avoid the confusions when (nebbie) user accidentaly changes the default blend method to something like "Divide". In this case the user will be wondering why nothing appears on the scren when he creates something. With this change the blend method will be reset to default value when application is restarted.
Use pixels as default unit of measurement (closes #710)

http://www.synfig.org/issues/thebuggenie/synfig/issues/710
Use "Frames" as default time format