added the function of drag and drop

[klengx]

NOTE: please paste this on your terminal for the program to work: pip install tkinterdnd2

• filename_label is updated when dropping a file and selecting other files in the listbox
• when saving a new file, that file is added to the listbox
• when opening a kkun file using 'open .kkun file' button, that file is added to the listbox