Enhancement: allow third party extensions in plugin.xml

[cmark]

Currently the code generator removes all extensions of the following extension points.

• org.eclipse.ui.menus
• org.eclipse.ui.handlers
• org.eclipse.ui.commands

Anyone, how contributes extension to one of these, loses it during Project->Clean build.

Main idea to solve this:

• prefix the extension id with additional information and use it to identify the removable extensions during Clean Build.

[cmark]

Fixed in master.