winebrowser: Fix broken handling of DDE type XTYP_EXECUTE

[Peter Dons Tychsen]

XTYPE_EXECUTE DDE transaction for winebrowser is broken.
The handler currently expects a MultiByte character input, which is
wrong.

The handler is win-unicode, and will always receive such strings.
The encoding of the string can be seen in WDML_BuildExecuteCommand().

This fixes programs which use DDE to show a web-link via winebrowser,
e.g. the "Last Chaos" web-installer:
http://lastchaos.aeriagames.com/download

Thanks,

/pedro