hgk: enable mouse wheel under Windows.
For some reason, MouseWheel events are not routed under windows even in latest ActiveTcl 8.4.15 while they are under linux and macosx. These events are activated using code supplied with Tcl Tip 171: <http://www.tcl.tk/cgi-bin/tct/tip/171.html>. Strangely, the Tip code almost work but generates some unexpected infinite loop which is fixed using a simple boolean to check reentrancy.
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