PluginForHost (Controlling): Add easy way for plugins to set "start one after another" mode
See workaround code in XFileSharingProBasic.
setStartIntervall is NOT the same than waiting until a download has really started before allowing the next one to start until max simultan limit for current situation is reached.
Especially if there is a high pre-download-waittime, StartIntervall will not help at all.
It is basically only usable for hosts with nearly instant downloadstart.