Allow users to modify dg_icon_template without PHP filter

[thenadz]

The PHP filter will still be available, but users will be able to avoid needing to write PHP in modifying the output structure.

This functionality will be exposed through the settings interface, possibly by way of a dialog where each of the special markup tags are available to insert by way of buttons (maybe along the top of the text box where the user can edit the HTML directly). Alongside the text editing box could be a live preview of what the output of the HTML & special tags would look like.

%link%: The URL that will be loaded when the user clicks the icon.
%target%: The target attribute for the anchor tag (e.g.: _blank, _self).
%img%: The URL pointing the the image that will be displayed.
%title%: The human-readable title of the attachment.
%title_attribute%: The escaped title (above), safe for using HTML tag attributes.
%description%: The attachment description (only present when rendering descriptions).
%extension%: The document file extension.
%size%: The human-readable file size formatted by size_format.
%path%: The system path pointing to the document.