Added new case to OperationDeserializer

Added the case of Plugins_Symbol in OperationDeserialize class

Related to issue karlitrotski/CodingSpectator/#12
Pair Programming with @karlitrotski
Code for get list of plug-ins moved

Code was moved to a new method named populatePluginsList() and this is
now called in populateTextChunk()rather than in constructor. This let
the constructor be called in OperationDeserializer class and create a
empty instance of PluginsList

Related to issue karlitrotski/CodingSpectator/#12

Pair Programming with @karlitrotski
Implemented initializeFrom() method

Fixes #12

Related to issue karlitrotski/CodingSpectator/#12

Pair Programming with @karlitrotski