Port WindowBuilder to GTK4

A lot of the native methods that are called on a Linux system are either deprecated or completely removed in GTK4. Meaning that trying to run WindowBuilder in such an environment will inevitably lead to a crash.

However, given that the Eclipse IDE crashes with GTK4, there is very little one can do right now. I'll try to have a look what exactly is broken and perhaps I can push some small fixes to the SWT project.