Only unload plugins after the DUChain shutdown to prevent infinite loop due to non-loaded factories for types

This is my workaround for this bug: https://bugs.kde.org/show_bug.cgi?id=379669

See the bug report for details. In short, the issue is that KDevelop sometimes goes into an infinite loop on shutdown, until the process is manually ended. This can be very bad for laptop users in particular if overlooked. The reason seems to be that some plugins may be unloaded before the DUChain is shut down, and then some type information is missing during shutdown which would be needed there.

This merge request thus moves the DUChain shutdown before the plugin unloading. While it is conceivable that some plugins will still want to access the DUChain on unload, it "works for me" (TM) based on using this change for a month, and thus it seems to be a clear improvement over the current situation. Maybe others could test too, then this nasty bug could hopefully finally be removed in case the workaround works for everybody.