change menu name from "Preferences" to "Settings" following current standard
initialize "Install on Internal Storage" pref at startup

This was only ever inited when the pref changed, otherwise it used the
hard-coded default.  Now it will read the value from the stored prefs on
startup, and the default value is a standard part of that call.
add preference to limit image file to 4GB (that's the FAT file size limit)

The "External Storage" is often a FAT filesystem, but it turns out that it
is very difficult to reliably detect what filesystem the External Storage
is using.  So this is a workaround using a manual preference.

fixes #80 https://github.com/guardianproject/lildebi/issues/80