Add "default actions" for each tags in the taglist

[Elv13]

Some tags could be designed to be "overlays"/"tooling" for other tags

use case:

• A tag to add/remove screen magnification
• A tag with multiple logs terminal
• Colors tools for web designer
• Tags with some "notop" floating clients

So, when clicking on the taglist, instead of doing a viewonly, it could either call a callback or do a predefined action like toggletag or some layout magic