BUG #15090: Fixed state loading + time issue.

The problem was that that vtkSMStateLoader was missing the timekeeper
proxy already present when loading state due to name mismatch. Fixed
that. Also cleaned up vtkSMStateLoader to use new/easier method on
vtkSMParaViewPipelineController to locate exisiting proxies of a
particular type.

Change-Id: I80e5f0778170118d2c43b4f63e8595190fbf037e
Merge topic '15090_fix_state_loading_issues' into next

f815af75 BUG #15090: Fixed state loading + time issue.