Add a way to unset the container runtime

Add a way to set the container runtime that Dangerzone uses back to the
default.

dangerzone/cli.py

@@ -52,7 +52,11 @@ def print_header(s: str) -> None:
 @click.option(
     "--set-container-runtime",
     required=False,
-    help="The path to the container runtime you want to set in the settings",
+    help=(
+        "The name or full path of the container runtime you want Dangerzone to use."
+        " You can specify the value 'default' if you want to take back your choice, and"
+        " let Dangerzone use the default runtime for this OS"
+    ),
 )
 @click.version_option(version=get_version(), message="%(version)s")
 @errors.handle_document_errors
@@ -69,6 +73,12 @@ def cli_main(
     display_banner()
     if set_container_runtime:
         settings = Settings()
+        if set_container_runtime == "default":
+            settings.unset_custom_runtime()
+            click.echo(
+                "Instructed Dangerzone to use the default container runtime for this OS"
+            )
+        else:
             container_runtime = settings.set_custom_runtime(
                 set_container_runtime, autosave=True
             )

dangerzone/settings.py

@@ -52,6 +52,10 @@ class Settings:
             self.save()
         return container_runtime
 
+    def unset_custom_runtime(self) -> None:
+        self.settings.pop("container_runtime")
+        self.save()
+
     def get(self, key: str) -> Any:
         return self.settings[key]