clean PLC log only if there is something to clean

Otherwise exception happens in Beremiz service.

Traceback (most recent call last):
File "/home/developer/WorkData/PLC/beremiz/beremiz/connectors/PYRO/_init_.py", line 113, in catcher_func
return func(*args, **kwargs)
File "/home/developer/WorkData/PLC/beremiz/beremiz/connectors/PYRO/_init_.py", line 194, in my_local_func
return RemotePLCObjectProxy._getattr_(attrName)(*args, **kwargs)
File "/usr/lib/python2.7/dist-packages/Pyro/core.py", line 381, in _call_
return self._send(self._name, args, kwargs)
File "/usr/lib/python2.7/dist-packages/Pyro/core.py", line 456, in _invokePYRO
return self.adapter.remoteInvocation(name, Pyro.constants.RIF_VarargsAndKeywords, vargs, kargs)
File "/usr/lib/python2.7/dist-packages/Pyro/protocol.py", line 457, in remoteInvocation
return self._remoteInvocation(method, flags, *args)
File "/usr/lib/python2.7/dist-packages/Pyro/protocol.py", line 532, in _remoteInvocation
answer.raiseEx()
File "/usr/lib/python2.7/dist-packages/Pyro/errors.py", line 72, in raiseEx
raise self.excObj
AttributeError: 'PLCObject' object has no attribute '_ResetLogCount'
+--- This exception occured remotely (Pyro) - Remote traceback:
Traceback (most recent call last):
File "/usr/lib/python2.7/dist-packages/Pyro/protocol.py", line 719, in _handleInvocation2
res = obj.Pyro_dyncall(req[1],flags,req[3]) # (method,flags,args)
File "/usr/lib/python2.7/dist-packages/Pyro/core.py", line 108, in Pyro_dyncall
return getattr(self.delegate or self,method) (args,*keywords)
File "/home/developer/WorkData/PLC/beremiz/beremiz/runtime/PLCObject.py", line 101, in ResetLogCount
if self._ResetLogCount is not None:
AttributeError: 'PLCObject' object has no attribute '_ResetLogCount'
+--- End of remote traceback

reset loaded messages counts on new PLC program trasnfer

This fixes the problem that first message from newer PLC program maybe
lost, if it's issued before first PLC log fetching.

Closes #23

add function block to variable panel if fb is added using toolbar

Closes #25