Kick off kbuildsycoca4 when updating sycoca5.

KBuildSycocaProgressDialog is used e.g. by the File Associations
KCM to perform a sycoca update after writing out a new
mimeapps.list file. mimeapps.list is intended to be evaluated at
query time, however the KDE 4 stack doesn't and instead relies
purely on sycoca being up to date. Since kded(4) doesn't keep a
watch on the file either it won't notice changes and update
sycoca by itself. Changing File Associations from System Settings
5 thus won't affect KDE 4 apps in reasonable time. This patch
addresses this problem by updating 4's sycoca at the same time
as 5's. The update is done via the same DBus-to-kded-or-
fallthrough-to-CLI-tool strategy, but non-blocking and without
any attempt to factor it into the progress display.

This is a workaround that should be removed if kdelibs4 gets
improved to evaluate mimeapps.list at query time at a later date.

Reviewed-by: faure@kde.org