Add "Clear Formatting" button in TinyMCE

We get our tickets via mail collector. Every user has its own format inside it's email. So every message looks different with different fonts and font sizes.

To get all messages look the same inside GLPI, I have to remove the format. TinyMCE has such an option (button) called "Clear Formatting".

Pasting content via [Ctrl]+[Shift]+[V] removes every formatting. Especially lists and so on. The TinyMCE function only removes bad font settings.