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
shell-global: Set high dpi scaling factor

Set the scaling factor based on the xsetting.

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