Add a blocking waitForKSycoca function

    When Kontact is started on a system where no kded was ever
    started before there is a race condition between kontact querying
    the KSycoca database and the creation of that database.
    This caused Kontact not load any plugins in that case.

    This Patch causes Kontact to block for up to 10 seconds while
    waiting for KSycoca to become available.

    BUG: 269807