This revives a bit of generate.sh that used to bundle in our own gsetting schemas into the AppImage. GTK expects certain keys to exist, so we ought to provide them.
Fixes #4817
This is basically a re-addition of commit c5907c68, which got removed in commit b2809175 (presumably by accident).
The apt_bundle gnome-settings-daemon-common bit was cargo-culted from Martin's original commit - I'm not sure what that was working around exactly. But it seemed harmless enough to include again.