Fixed bug get_current_item w/o grid, fixed bug when reading pnml.

-> _get_current_item function in PNEditor would always get an item when clicked in the empty workspace when there is no grid.
-> Reading from pnml files was buggy and getting the default values:
  '-> I was querying toolspecific[@name] instead of @tool.
  '-> Many find('<XPATH>/text').text calls missed the .text at the end to read the actual property.
-> Moved "add" lines of code of the mainWindow to a function "_add_pne" which is called by create and import functions.