Conditionally putting a "Who's bookmarked this" tab on a node

[This is an offshoot of #209371, Optionally show users who've bookmarked a given node.]

Building a "Who's bookmarked this" tab is easy; however, in conditionally putting this tab on a node we need some help from Views.

Overall, there are four solutions:

1. Views already provides the node/$node-event/who syntax, which allows us to put the tab on, e.g., event nodes only.
2. I've just proposed an extension #211093 to this syntax, to allow us to specify several node types. (Please lend your '+1' there ;-)
3. An alternative solution: we can introduce our own '$bookmarkable' token, which serves a similar purpose. See the patch.
4. quicksketch envisions the following feature:
   [ x ] Show tab on node page for users who marked this content
   And there's an interesting way to solve this: besides '$bookmarkable' let's have a '$bookmarked-by-user' token too. This token will match only nodes that were bookmarked by the user browsing the page.