ArchiveDefinition: some fixes for Windows (primarily):

• allow commands with spaces in the name by using KShell::splitArgs()
• this requires us to escape the %f marker with %PERCENT_ESCAPE%f, lest slitArgs misparses it
• in addition, use findExe() to check that the executables do, in fact, exists (and to work around the problem that, apparently, QProcess doesn't use PATH...)

svn path=/branches/kdepim/enterprise4/kdepim/; revision=1088345