Debugger: Consolidate some GDB-through-Python interaction

Instead of calling a GDB extension command invoking a function
on the well-known Dumper instance, call the Dumper function
directly. This also makes the code more similar to the LLDB side.

Change-Id: I4b23177eb72a904721b63c578ce7fbfe204f02a2
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>