Adopt tokens from workflow_extensions

Problem/Motivation

The module workflow_extensions provides two tokens:
- token 'workflow-state-age' = 'Time elapsed since last state change (in seconds)'
- token 'mod-since-seconds' = 'Seconds elapsed since last modification of node'

Since workfow_extensions functionality is now being absorbed in workflow 7.x-2.3, please provide these tokens.

Proposed resolution

(Description of the proposed solution, the rationale behind it, and workarounds for people who cannot use the patch.)

API changes

The token '[mod-since-seconds]' is already available in D7 as'[node:changed:since]'
The token '[workflow-state-age]' (that shows the time in seconds since last state change) from Workflow Extensions is now named as [node:last-transition:created:seconds].
See also #2240009: Multiple workflows per bundle/node_type & tokens