[ExeFramework] Refactoring: move debugger code into new dedicated plugins

[ebousse]

(This should not be merged now, as it will break concurrency and coordination. I'll prepare merge request on these other repos as well, to merge them at the same time. But you can already have a look and comment)

These commits aim at extracting all the debugger-related code of the execution framework into new dedicated plugins:

• org.gemoc.executionframework.debuggerwith the business logic of the debugger (for now still polluted by a small amount of UI code)
• org.gemoc.executionframework.debugger.uiwith all UI related code of the debugger (eg. breakpoint view, context menus, etc.)

All the moved code was previously mostly inside the plugin org.gemoc.executionframework.engine.ui.

The goal of this refactoring is to facilitate the next tasks, in particular https://github.com/gemoc/gemoc-studio/issues/44, https://github.com/gemoc/gemoc-studio/issues/45 and https://github.com/gemoc/gemoc-studio/issues/46

[ebousse]

PR for concurrency ready here: https://github.com/gemoc/concurrency/pull/9

@dvojtise If this looks OK to you, you can merge :)