Four years later, and nobody really elaborated on this.

From a consistency standpoint, packing a button into a tab widget should not cause wildly different results. I fail to understand what adapting to different devices would entail, but that's generally what gestures are for.

Closing tabs from a toolkit is also fairly dangerous: the toolkit lacks all the information needed to, for instance, undo an operation, or ask for a confirmation, or simply understand what the contents of the tab are.

I strongly believe this is application territory, and as such it's the remit of the application to add a close button to the tab widget, and handle its behaviour.