GtkStatusIcon/w32: Misc fixes for the handling of taskbar_created_msg

The first commit fixes #2030 (closed).

The problem is in this line: gtkstatusicon.c:747. By resetting the flags to NIF_MESSAGE (without NIF_TIP) we loose the tooltip. The tooltip is important because Windows uses that to store and apply user preferences on individual taskbar icons. For an explanation you can read: https://bugzilla.gnome.org/show_bug.cgi?id=609622

The second commit, instead, fixes the following issue:

1. Create a GtkStatusIcon
2. Call gtk_status_icon_set_visible(icon, FALSE) to hide the status icon
3. Either:
   • WinKey+X > TaskManager > Terminate explorer.exe > File > Exec > Type explorer.exe > Ok
   • Change DPI
4. After that the status icon is present in the taskbar

What should happen: the status icon is hidden so it should not appear in the taskbar!
To test this you can compile the C sample: statusicon.c