st: Add high dpi support

Add a scale_factor property to StThemeContext that can
be used to enable (integer) scaling of pixel values.

https://bugzilla.gnome.org/show_bug.cgi?id=705410
layout: Set high dpi scaling factor

Set the scaling factor when the dpi is above the hidpi threshold.

https://bugzilla.gnome.org/show_bug.cgi?id=705410