Improve criteria handling

This commit changes how commands decide what container to act on.
Commands get the current container though `current_container`, a global
defined in sway/commands.c. If a criteria is given before a command,
then the following command will be run once for every container the
criteria matches with a reference to the matching container in
'current_container'. Commands should use this instead of
`get_focused_container()` from now on.

This commit also fixes a few (minor) mistakes made in implementing marks
such as non-escaped arrows in sway(5) and calling the "mark" command
"floating" by accident. It also cleans up `criteria.c` in a few places.

sway/commands/focus.c

@@ -30,6 +30,9 @@ struct cmd_results *cmd_focus(int argc, char **argv) {
 			}
 		}
 		return cmd_results_new(CMD_SUCCESS, NULL, NULL);
+	} else if (argc == 0) {
+		set_focused_container(current_container);
+		return cmd_results_new(CMD_SUCCESS, NULL, NULL);
 	} else if ((error = checkarg(argc, "focus", EXPECTED_EQUAL_TO, 1))) {
 		return error;
 	}