Cut with fading the Bar title when it's longer than the decoration bar

With this patch, when an application title is longer than the decorator
available space for title, the text is colored using a cairo linear pattern
which is divided into two parts:
 * The first pixels - apha_margin are colored with the defined opaque color
 * The pixels from alpha_margin until the string end are colored using
   a cairo pattern going from the opaque color to the transparent one.
Update previous commit for gtk+-3
Previous patch was made for gtk+-2 version. Update it to work with
gtk+-3 version.