feature request: try 7zip for unknown archive formats

[ilyapashuk]

engrampa version: 1.26.0, archlinux with latest updates

engrampa uses 7zip for 7z archives only, but 7z supports many more archive and container formats.

how to reproduce the behavior:

1. run something like this commands:
   dd if=/dev/zero of=t.img bs=1M count=10
   mkfs.vfat t.img
   sudo mount -o loop t.img /mnt
   echo 'hello, world' > /mnt/hello.txt
   sudo umount /mnt
2. you will get a small disk image with vfat filesystem.
3. try to open it in engrampa. it should not open, which is expected.
4. and now rename it to "t.img.7z". and try to open it again.
   if 7zip is installed, it should open as if it was a 7zip archive.
   it works for squashfs images, many virtual machine images, initramfs files and in many more cases.
   wanted behavior:
   if Engrampa founds an archive with unknown format, it should try to open it with 7zip before brinning up the error window.

on Windows it is very convenient to open virtual machine images, linux filesystem images and a lot more container types with 7zip gui filemanager.