Migrate away from the deprecated UI extensions

In several of our plugin.xml files, we use the long-deprecated extension points:

- org.eclipse.ui.editorActions
- org.eclipse.ui.actionSetPartAssociations

https://github.com/eclipse-windowbuilder/windowbuilder/blob/d650c204da1c627c89592c11a57668bdc5d5ded0/org.eclipse.wb.core.java/plugin.xml#L342-L354


We should try to move away from them using the following replacement extension points:

- org.eclipse.ui.menus
- org.eclipse.ui.bindings
- org.eclipse.ui.commands
- org.eclipse.ui.handlers

If possible, one might even consider using the e4 XMI fragment instead of the extension points.