set robots tag value as placeholder instead of inserting code

Inserting code into the document is "not the MODX way"... Also ```regClientStartupHTMLBlock``` didn't work on ```OnHandleRequest```.

Switched to event ```OnLoadWebDocument``` and inserting now a Placeholder ```[[+seoTab.robotsTag]]``` that includes the value and could be integrated into the template like
```
<meta name="robots" content="[[+seoTab.robotsTag]]">
```
...thats creative freedom for the developer :)

Note: do we need to add any script that changes the event during a update? Or is this done automatically by the changes in build script.

ToDo for freedom:
The default value (Line 109) should be editable through a system setting.