Click passes element

**WIP: Spec work remaining**

For all but the application and slots, we actually want click handlers
to pass along the element clicked, not the mouse information (button, x,
y). Addressing this fixes #837.

This commit does the production work to bake in the necessary parameter
for it to the `Shoes::Common::Clickable` module, and wires it up for
proper behavior in the SWT backend.

This only adds the spec to `TextBlock` and leaves the few that had it
before for the shared example "clickable backend". This shared example
should definitely be applied more broadly, so this isn't ready to merge yet.

Also cleans up a small rough edge in `Shoes::Common::Clickable` where
we had multiple methods for setting a click, at least one of which was really
a duplicate that's now been folded into the prior implementation.