Cleanup and simplify token generation and replacement

In the participant pdf generation, a nice addition would be to add event and participant tokens.
This would also be handy to have in participant mailing.
Event tokens already exists in scheduled reminders.

However those tokens get generated and repaced on very different ways. To improve DX, and code debt, this needs an 'overhaul'.

Plan of approach. (Todo in order)
-Cleanup the form to add tokens https://issues.civicrm.org/jira/browse/CRM-16706
-Cleanup add tokens to a form https://issues.civicrm.org/jira/browse/CRM-16707
-Add event tokens to participant pdf creation. This is a first approach to a 'cleaner' token usage. https://issues.civicrm.org/jira/browse/CRM-16599
-Add event token to participant mailing. Same approach of above. https://issues.civicrm.org/jira/browse/CRM-16579
-Replace event tokens in scheduled remainders with the Token class functions.

Following steps needs some more fine grained plans. However general tasks should be something like this.
-Move tokenlists from SelectValues class to Token class.
-Replace hardcoded tokens with general getFields calls, and extra tokens.
-Generalize on token-replacement method
-....

Original report:
When going to the https://github.com/DizzyMissLizzy/be.dizzymisslizzy.pdfletterevents extension, a hidden treasure is the addition of event and participant tokens.
These could be of good use in the email (and print pdf) actions.