text: Title: Add library, resource file and variable file import related methods to listener version 3
  Body: Listener version 3 was initially implemented in RF 3.0 (#1208). It didn't have `start/end_keyword` methods nor methods related to library, resource file and variable file imports because implementing them would have been too big task. It finally got keyword and control structure related methods in RF 7.0 (#3296), but it still lacks methods related to library, resource file and variable file imports.
  
  Listener version 2 has methods `library_import`, `resource_import` and `variables_import`, and we probably should use same name with listener version 3 as well. With listener version 2 all these methods the name of the imported thing and a dictionary containing other information as arguments. With listener version 3 we want to pass the real model objects instead similarly as we do with `start/end` methods. Listeners should be able to modify at least resource files but preferably also libraries and variable files.
  
  The good news is that implementing `start/end_keyword` methods required refactoring also library and resource file related model objects and they both ought to be in good enough shape to be used directly. With variable files we don't have such a good model object yet, but creating one shouldn't be too complicated. If it turns out to be hard, just adding methods for libraries and resource files and delaying adding methods for variable files is fine as well.
  
  I consider this pretty high priority because after that listener version 3 can handle everything that listener version 2 can.
  
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  **UPDATE:** We won't create model objects representing variable files in RF 7.1, but the variable file related method will be added anyway. It will simply get a dictionary with same information as the listener v2 method, and the dictionary will be replaced with a proper object later when such an object is available. That change is covered by #5116.
  