Use combobox instead of switch for Filesystem permissions

[sonnyp]

Filesystem permissions are not binary. There are 3 options, ro, rw (default) and create.

I propose to replace the GtkSwitch with a GtkCombobox to allow for one of of the following options Off / Read write / Read only / Read write create

I think it would encourage users to use Read only as well since it's not the default option.

It took me a while to understand why filesystem=host wasn't active for my app.