patch 9.1.1583: gvim window lost its icons

patch 9.1.1583: gvim window lost its icons

Problem:  Since patch 9.1.1199 the gvim window no longer had _NET_WM_ICON
          nor WM_HINTS icon information, for example when not using a
	  Gnome or KDE desktop (after v9.1.1199)
Solution: Check if the icon theme as used in patch 1199 contains a gvim
          icon. If so, set the window's icon from that. Otherwise
	  use the previous method (Olaf Seibert)

fixes: #17703
closes: #17814

Signed-off-by: Olaf Seibert <rhialto@falu.nl>
Signed-off-by: Christian Brabandt <cb@256bit.org>