Fix broken control panel icon (Plone uses PNG now)

The icon in controlpanel.xml is for Plone 4; actionicons.xml is used for Plone 3, which has only GIF images, so I've left that alone.  The action_icon.gif file in this product is not used at all, so remove it.

GIF images like action_icon.gif were in the plone_deprecated skin in 4.x; this layer is no longer active by default in 4.3, and is removed completely in HEAD (and the future Plone 5).

Tested against Plone 3.3, 4.1, and 4.3.

I've also added an appropriate gitignore file.