Found a 'clean' workaround [tested on Ubuntu 13.04 (amd64) & Eclipse Kepler]
As it seems, GWT (also SWT) needs an older libwebkitgtk1.0 to run the designer.
So, assuming you already have the 13.04 Ubuntu repository 'libwebkitgtk1.0' (v.1.10.x) installed (as a dependency of Gimp for example), and don't want to mess with it:
1) Download the following (from launchpad.net) - BUT DO NOT INSTALL THEM:
2) Open the 2 files downloaded with Archive manager
3) Create folder $HOME/.eclipse/lib
4) Extract the 'lib' folders contents of the 2 deb files in the folder created
5) Launch eclipse with the following command line (from within the eclipse folder):
6) Switch to the designer tab - should work.
Basically, what you do is forcing eclipse to first check '$HOME/.eclipse/lib' folder for any needed lib and later the /usr/lib - so you place there the older libs and picks them instead of the system installed ones (and the system is still clean/untouched)
(Just a notice - this is not a fix - its a workaround)