Win32: Re-work cursor handling

Win32: Re-work cursor handling

Like the X11 and Wayland backends, re-work how the cursors are being
handled.  So, we use a hash table to cache up the HCURSORS that we
create along the way.

We still need to cache up the icon/cursor themes since this is something
that is not part of Windows but was added on to support icon/cursor themes
such as Adwaita on Windows, but should be in-line with what is going on in
GdkCursor.

Also, remove the _gdk_grab_cursor global variable in gdkprivate-win32.h,
and replace it with another variable in the GdkWin32Display structure,
to make things cleaner in the process.

https://bugzilla.gnome.org/show_bug.cgi?id=773299