Implement icon_pixmap and icon_mask from WM_HINTS (FS#1297)

Fun fact: ICCCM specifies that icon_pixmap must have depth 1. Xterm uses a
pixmap with depth 24. Yay... As such, I don't have any test for the depth == 1
case and will just assume that it does the right thing. If it doesn't, I bet no
one will notice anyway.

Signed-off-by: Uli Schlachter <psychon@znc.in>