debugger: add minimal debugger API
This is an initial implementation to provide a debugger API to Builder. It is not complete, but should be considered a technology preview as we complete the feature-set that users come to expect from debugger tooling. It should, however, give you an idea of where we are going with things, even if UI and APIs should change a bit in the upcoming months. Debuggers should implement the IdeDebugger API (which is a base class to make things more convenient). A reference implementation using GDB will be provided in a forth-coming commit. Most operations are asynchronous, as debuggers can be very slow given some hardware configurations. Of note, we do not yet have support for diving into varibles such as fields, variable hover, or data visualizers. These features and more avanced ones such as server-side variables will come in the form of a future release, such as GNOME 3.28.
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