Import stylesheet code from hippo-canvas

Import:

  HippoCanvasTheme      => StTheme
  HippoCanvasThemeImage => StThemeImage
  HippoCanvasStyle      => StThemeNode

StThemeContext is a new class managing the theme for a stage and
global properties like resolution.

test-theme.c is a newly written test program to do verification of the
style matching and property handling rules.

Various changes are made in the import:

 - Comprehensive reindentation
 - guint32 pixels replaced with ClutterColor
 - General pseudo-class support added
 - Old-fashioned (non-bordered) background image support added, though
   with no support for repeat, etc.
 - Bug fixes for problems revealed by test program

https://bugzilla.gnome.org/show_bug.cgi?id=595990