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/debug_session.py

@@ -528,7 +528,6 @@ class DebugSession( object ):
     stack_trace_window = vim.current.window
     one_third = int( vim.eval( 'winheight( 0 )' ) ) / 3
     self._stackTraceView = stack_trace.StackTraceView( self,
-                                                       self._connection,
                                                        stack_trace_window )
 
     # Watches
@@ -546,17 +545,15 @@ class DebugSession( object ):
     with utils.LetCurrentWindow( stack_trace_window ):
       vim.command( f'{ one_third }wincmd _' )
 
-    self._variablesView = variables.VariablesView( self._connection,
-                                                   vars_window,
+    self._variablesView = variables.VariablesView( vars_window,
                                                    watch_window )
 
     # Output/logging
     vim.current.window = code_window
     vim.command( f'rightbelow { settings.Int( "bottombar_height", 10 ) }new' )
     output_window = vim.current.window
-    self._outputView = output.OutputView( self._connection,
-                                          output_window,
-                                          self._api_prefix )
+    self._outputView = output.DAPOutputView( output_window,
+                                             self._api_prefix )
 
     # TODO: If/when we support multiple sessions, we'll need some way to
     # indicate which tab was created and store all the tabs