enable entering a new directory in the current directory combobox * main_window.cc: construct (): use QLineEdit for the line edit in _current_directory_combo_box, connect its signal returnedPressed () to new slot current_working_directory_entered (), use const int instead of hard coded constants for combobox parameters; set_current_working_directory (): Check whether directory exists; new slot current_wirking_directory_entered (): set current directory to the entered one if it exists * main_window.h: new slot current_working_directory_entered (), define constants for combobox parameters, QLineEdit _current_directory_line_edit to be used in combobox

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