improved handling and displaying dropped level set zip files into window

This change added the possibility to drop and extract zip files into
the currently displayed level set sub-directory on the "choose level
set" screen.

After dropping and extracting zip files on the "choose level set" or
"choose custom artwork" screens, the newly added level or artwork set
is displayed and pre-selected in the list of level or artwork sets,
so it can easily be loaded. (When dropping and extracting zip files
with level sets on the main menu screen, the last added level set will
be automatically selected and loaded.)