GTK+ 3 build of the LXDE packages

Experimental up to date GTK+ 3 build of LXDE packages for personal use.

USE AT YOUR OWN RISK!

Known bugs:

- #773 [lxpanel] Pager applet breaks panel layout with GTK+ 3
  https://sourceforge.net/p/lxde/bugs/773/

- [lxpanel] Taskbar plugin makes the panel grow beyond the screen width

- [lxpanel] "lxpanelctl run" dialog input field height is too big (it has the height of the icon)

- #768 [lxappearance-obconf] Broken preview rendering with GTK+ 3
  https://sourceforge.net/p/lxde/bugs/768/

- #769 [gpicview] Enlarged images are not displayed correctly on GTK+ 3
  https://sourceforge.net/p/lxde/bugs/769/

  Pending fix: #542 [gpicview] Fix displaying images with GTK3
               https://sourceforge.net/p/lxde/patches/542/