Add the category option to the widget's form

gistfile1.aw

<p>
	<label for="<?php echo $this->get_field_id('title'); ?>"><?php _e('Title'); ?>: </label>
	<input type="text" id="<?php echo $this->get_field_id('title'); ?>" name="<?php echo $this->get_field_name('title'); ?>" value="<?php echo $instance['title']; ?>" />
</p>
<p><!-- this adds the category option to the widget's form -->
	<label for="<?php echo $this->get_field_id('category'); ?>"><?php _e('Category ID','dbem'); ?>: </label>
	<input type="text" id="<?php echo $this->get_field_id('category'); ?>" name="<?php echo $this->get_field_name('category'); ?>" size="3" value="<?php echo $instance['category']; ?>" /><br />
	<label for="<?php echo $this->get_field_id('category'); ?>"><?php _e('(separate multiple IDs by commas, or use "0" for all categories)','dbem'); ?> </label>
</p>
<p>
	<label for="<?php echo $this->get_field_id('limit'); ?>"><?php _e('Number of events','dbem'); ?>: </label>
	<input type="text" id="<?php echo $this->get_field_id('limit'); ?>" name="<?php echo $this->get_field_name('limit'); ?>" size="3" value="<?php echo $instance['limit']; ?>" />
</p>