First methods on app-level click listener
Wire-up for SWT event listeners
Handle clicks and releases
Support removing an element's listeners
Small extraction and renaming
Don't allow click on hidden elements
Enable option to pass coordinates on click
Switch over to new app-level ClickListener
Don't need to pack SWT element around, just pass DSL
Don't respond on non-mouse events
Clarifying comment on why we choose last
Link wasn't calling clickable, so it wasn't used

Bye bye!
Set up basic click_listener in swt app context

This prevents many tests from needing their own special click support,
except where they're testing clicking behaviors more rigorously
themselves.
Don't do respond_to? check on :hidden?

If there's a problem with this in an app, will find it working through
samples, but otherwise everyone should respect the visibility methods.
App needs hidden? for click handling
Merge pull request #980 from shoes/one-click-listener

Use app click listener to only send click to one element

Fixes #882

Changelog: bugfix