lua : add style management

darktable.styles.# : table of styles
darktable.styles.create(image,name,description) : create a new style based on image, return the created style object
darktable.styles.delete(style) : deletes a style
darktable.styles.duplicate(style) : duplicate a style
darktable.styles.apply(style,image) : parameter order can be changed, apply the style to the image

<image>:create_style(name,[description]) : see darktable.styles.create
<image>:apply_style(style) : see darktable.styles.apply

<syle>.name : the name of the style
<style>.description : the description of the style
<style>:duplicate(image) : see darktable.styles.duplicate
<style>:delete(image) : see darktable.styles.delete
<style>:apply(image) : see darktable.styles.apply