aafm-gui: Avoid locking up during long-running process

This isn't ideal, as recursive directory copying isn't
exposed as per-item progress, but this can be fixed later.
execute: Use file.readlines() instead of doing it manually
Don't use shell and quoted strings, execute as-is
Combine parse_device_list and device_list_files
Fix a directory type detection bug

When /mnt/sdcard is a symlink, checking if /mnt/sdcard/DCIM (for
example) is a directory must list "/mnt/sdcard/", not "/mnt/sdcard".
GUI: Make the queue actually be a queue and not a stack
device_make_directory: Only create directory when needed
Recursive copying and deleting functions with progress
Fix panels paned width calculation with newer Gtk versions
Context menu "copy to computer" should re-use existing logic
Main menu + refresh view menu item
Support for multiple devices and show free disk space
Refresh device list menu item
Better handle "no devices found" case
Merge pull request #59 from thp/fixes0

Various fixes and improvements