Add ARGB visual support.

Add ARGB visual support.

This patch adds the possibility to use a argb visual for the conky
window.  Adding a boolean configuration entry 'own_window_argb_visual'.
the 'set_transparent_background' method sets the alpha bytes of the argb
color for transparency, instead of pseudo transparency.  This patch
would close following feature request:
https://sourceforge.net/tracker/?func=detail&aid=1563931&group_id=143975&at
id=757311 and would be a workaround for some reported transparency bugs
(https://sourceforge.net/tracker/?func=detail&aid=1968923&group_id=143975&a
tid=757308 for example)

Signed-off-by: Brenden Matthews <brenden@diddyinc.com>