[try 3][systray 1]: explorer: make null window background

[Kirill K. Smirnov]

Please, forget my previous bunch of systray patches.
Use this one.
Sorry.
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diff --git a/programs/explorer/systray.c b/programs/explorer/systray.c
index 7fc66fd..7ffd81c 100644
--- a/programs/explorer/systray.c
+++ b/programs/explorer/systray.c
@@ -433,7 +435,7 @@ void initialize_systray(void)
     class.hInstance     = NULL;
     class.hIcon         = LoadIcon(0, IDI_WINLOGO);
     class.hCursor       = LoadCursor(0, IDC_ARROW);
-    class.hbrBackground = (HBRUSH) COLOR_WINDOW;
+    class.hbrBackground = NULL;
     class.lpszClassName = adaptor_classname;
     class.style         = CS_SAVEBITS | CS_DBLCLKS;