Set window_created to 0 in destroy_window

Since other code now depends on this value (core.c checks the flag
before dealing with colors), it should be properly unset, otherwise
conky-ng crashes with an X Error.
Revert "Use the alpha of the current color for Xft"

This reverts commit acab93c927ffadc0cbe5561889d5926708c31151.
This is necessary because Xft uses the alpha component even if no 32-bit
visual is available (it does pseudo-transparency in that case). The
alpha component cannot be stored in the color in get_x11_color if no
32-bit visual is available, as that will result in X errors.
Use the alpha of the current color for Xft

This is the same as acab93c, but only uses the alpha component in case
a 32-bit visual is available. In other cases the xftalpha value is used.