Allow 'separators' to be included in plugin_reg.php

As of version 4.7.0 I could organize my buttons by editing plugin_reg.php, but due to some limitations in tinymce.module it was impossible to add separators in between them.

This simple patch fixes that problem and allows one to add 'separator' pseudo-buttons to plugin_reg.php.

It'd be nice if one of the authors could review and commit my changes. I had to comment out a line of code that would not allow repeated buttons in the array. It hasn't brought any side-effects as far as I can tell...