Handeling plugin loading in python tracer.

Python tracer did not load plugins when loaded from GUI. This
was because the tracer did not handle any events for plugin loading.

Two new events named LocalPluginLoadedEvent and RemotePluginLoadedEvent
were added and triggered when LocalPlugins and RemotePlugins we
loaded. Handelers for these evens were added in vtkSMPythonTraceObserver
and consequently in smtrace.py.

on_local_plugin_loaded() and on_remote_plugin_loaded() in smtrace.py
write out the corresponding pythong code to the tracer output.

Change-Id: I85942da954e71273ec03a6a3d493965f22d70119
Merge topic '0012052-python-trace-load-plugins' into next

72bfe4a Handeling plugin loading in python tracer.