* Introduced two new attributes of MainWindow:
cur_img_idx - Index of current image in m_img_list.
img_count - Length of m_img_list.
* Implement counter_str method to get string representation of counter.
* Appended counter to window title.
* Based image switching logic on counter instead of local variables.
* Removed useless condition.
current_index = self.m_img_list.index(ustr(item.text()))
if current_index < len(self.m_img_list):
list.index will raise ValueError if item doesn't exist.
* Fix incorrect save dir path handling for ML format.
Save dir path was split by incorrect os.dependent separator '/'
which was the cause of labels being saved to parent folder.
* Implement path normalization for command arguments.
Renamed 'predefined_classes_file' parameter to 'class_file'.
Denis