Use more appropriate icon for "Create New" action

Right now this action is using the "document-new" icon, which depicts
creating a new file. This is inaccurate, as the menu provides options for
creating more than just files: it has one option to create folders and
three for creating different types of links. In fact, depending on the
set of installed software, there may not be any options to create new
documents at all!

To fix this, the icon would need to be made either more specific or
more generic.

There is no such icon that depicts being able to create a folder, a
document, or a link; such a thing is too specific to be able to
represent with a small icon. So our only real option is to use a more
generic icon.

This commit does that.