New <Back>, <Cancel> & <Forward> tags (to be contained within <Form>) for control configuration:
 - these tags can have the following attributes: img, backgroundColor, desc/description
 - control configuration is now represented by instanced of the new Control class (of which Form has an instance for each control type: Back, Cancel & Forward)
 - *removed* support for parsing the following <Form> attributes (as far as I know they were never used in v1.x projects): forwardButtonImg, cancelButtonImg, backButtonImg, forwardButtonDesc, cancelButtonDesc, backButtonDesc
 - <Form> attribute buttonBackgroundColor is still parsed for v1.x compatibility and has a new alias called controlBackgroundColor, either of these can be used to set the default background color for controls which do not specify their own color, and the background color of the photo capture button
 - implemented long click audio feedback for controls

Improvements to parsing & using of (audio) descriptions. New Description class bundles textual and audio descriptions.

Changed the getFiles() method on Form & Fields to addFiles(), taking a Set<File> to add to. Also added missing entries for audio feedback related files.

Signed-off-by: Matthias Stevens <matthias.stevens@gmail.com>