change bar colors from char[10] to *char

This commit removes has_* booleans from bar color struct. It also
generalizes of functions in commands/bar/colors.c.

sway/commands.c

@@ -255,6 +255,7 @@ struct cmd_results *add_color(const char *name, char *buffer, const char *color)
 		buffer[7] = 'f';
 		buffer[8] = 'f';
 	}
+	buffer[9] = '\0';
 
 	return NULL;
 }