Switch to running the actual install_gadget.py

This re-uses the OutputView code to run the installer script. Refactor
to remove connection from the base OutputView (and other places, it
wasn't used - only used after ConnectionUp).

This also consolidates the stdout and stderr buffers for running jobs.
The distinction was always arbitrary and probably an error, based on the
fact that they were separate in the APIs not based on usability.

python3/vimspector/stack_trace.py

@@ -21,13 +21,13 @@ from vimspector import utils
 
 
 class StackTraceView( object ):
-  def __init__( self, session, connection, win ):
+  def __init__( self, session, win ):
     self._logger = logging.getLogger( __name__ )
     utils.SetUpLogging( self._logger )
 
     self._buf = win.buffer
     self._session = session
-    self._connection = connection
+    self._connection = None
 
     self._current_thread = None
     self._current_frame = None