[MWF] Fix hang if clipboard source application closes down (fixes #674098)

This change adds a timeout to the Clipboard.Enumerating loop, making a
hang less likely. This fixes Novell bug #674098
(https://bugzilla.novell.com/show_bug.cgi?id=674098).
Merge pull request #926 from ermshiperete/novell-bug-674098

[MWF] Fix hang if clipboard source application closes down