Ui Retool

Retooled the Parasite window:
- moved the buttons to a toolbar
- replaced the 'Edit Mode' button with click-able column headers for Realized/Mapped/Visible
- added a right-click popup menu to the column headers to show/hide columns
- put the Properties pane in a notebook with vertical tabs on the right
- added a class hierarchy tab
- added a WidgetPath tab (Gtk+3-only)

Screenshot: 
![gtkparasite3_0pjh2-screenshot](https://f.cloud.github.com/assets/3309460/79766/8f547e86-61c7-11e2-9e69-f702636eb5c5.jpg)

Important note: the Gtk+3 features are dependent on the 'GTK3' conditional define added in the bug-fixes pull request.