archive bug fixes

- images in directories within archives are now displayed and cleaned up
propery
- 7z can now be called from the global path rather than just through
7z.exe (ie - should work on unix-like systems if 7z is installed)
- if 7z is not found, archives will not be opened
- if 7z is not found, archives will not show up in the open file dialog
- empty directories/archives will no longer cause the application to
crash
- misc cleanup