Text in Input per Klick hinzufügen, bei erneutem Klick Text löschen

HTML-Quelltext

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 
<html> 
<head> 
<title>Formular mit BBCodes</title> 
<script language="JavaScript" type="text/javascript"> 
<!-- 
function text(Text) { 
document.formularname.textfeldname.focus(); 
document.formularname.textfeldname.value = 
document.formularname.textfeldname.value + Text; 
document.formularname.textfeldname.focus(); 
} 
//--> 
</script> 
</head> 
<body> 
<h1>Formular mit BBCodes</h1> 
<form name="formularname" action="action.php" method="post"> 
<textarea name="textfeldname" cols="50" rows="3"></textarea> 

<button type="submit">Ab damit!</button> 
<button type="reset">Nööö!</button> 
</form> 
<a href="javascript:text('[B]fett[ /B]')">fett</a> 
</body> 
</html>