Add buttons to the GUI that will cycle to the Next or Previous detection files.  Also minor tweak to how the currentcall is selected when >>> or <<< is used at the end/beg of file.

Add Prev/Next File buttons to figure file

Changed 'handles.current_detection_file' variable from just the file name to the full file path
Added function to check if they file was changed and prompt to save when new calls or audio are loaded
Removed unnecessary handles variable in 'savesession_Callback'
Fixed bug where saving the session after loading an audio file would use the previously loaded detection file as the default file name.

…ave changes