bash-completion: add completion for gstreamer tools

Allow to autocomplete with elements/properties in the devenv using gst-launch or gst-inspect.

gst-env.py

@@ -518,7 +518,8 @@ if __name__ == "__main__":
             tmprc.flush()
             bash_completions_files = []
             for p in BASH_COMPLETION_PATHS:
-                bash_completions_files +=  os.listdir(path=p)
+                if os.path.exists(p):
+                    bash_completions_files +=  os.listdir(path=p)
             bc_rc = BC_RC.format(bash_completions=' '.join(bash_completions_files), bash_completions_paths=' '.join(BASH_COMPLETION_PATHS))
             tmprc.write(bc_rc)
             tmprc.flush()