When the Plug-in view is enabled on startup UI freezes since the
initialization of the TP state causes some time. It is uncritical to run
this in the background. Once the PDEState has finished computing the
SourcePluginFilter is added to the viewer and refreshes it with the new
filter.

Change-Id: I2043c9072c066c829a200b44c523b959eff7572b
Signed-off-by: Karsten Thoms <karsten.thoms@itemis.de>