Support for multiple frame icons
Merge pull request #130 from ummels/icon-images

Support for multiple frame icons
Support for :listen option when constructing a frame
Expand on how :on-close affects events fired by frame.
modified :import option on default-transfer-handler to allow checking of data before accepting import

:import

can now accept either a function (as before)
or a map {:on-drop   (fn {data] ...)
          :can-drop? (fn {data] ...)}
Merge branch 'dnd-filter-2' of https://github.com/davesann/seesaw into develop
Fix implementation of value! for nil values

The implementation of value! was incorrectly filtering out falsey values
which caused things like clearing text from a nested text box to fail.
Thanks to @geraldodev for pointing this out.