Allow to add Customize the UI of Debug.UI's WorkingDirectoryBlock

[azoitl]

As part of

• https://bugs.eclipse.org/bugs/show_bug.cgi?id=214696
• https://bugs.eclipse.org/bugs/show_bug.cgi?id=221973

the WorkingDirectoryBlock was mad public API so that the code in this class does not have to copied around.

The main task of this code is to provide a launch configuration tab for setting the working directory for a launch configuration. This is very handy for many different launch configurations.

However in its current form it will provide a tab with only one main widget for setting the working directory. A typical use case for this abstract base class would be to add more widgets for specific launch configurations. However this is currently not possible as the createControl() is final.

I would like to propose to remove the final from createControl(). As this would give users of that class the flexibility to add widgets before and after the working directory widgets.

If this proposal is accepted I'm happy to provide the according PR for it.