Add conditional skin attribute to check if dictonary exists

This makes it possible to hide e.g. color buttons in templates when they
are not required. And also remove some warnings if templates are used.
See example for red button for the scale PLiHD skin below

<screen name="redbutton">
 <ePixmap pixmap="PLi-Scale-HD/buttons/red.png" position="left" size="26,*"
  alphatest="on" conditional="key_red,red_key,red,red_text,canceltext,Delete" />
 <panel position="left" font="Spacer" size="2w,*" />
</screen>