Add "Allow SSH/SFTP access" option to File Manager's menu

[Peter Bittner]

For doing file transfers entirely via WiFi, without USB cable, apps exist, such as WifiTransfer [1]. Their abilities are restricted due to the confined nature of apps. However, it is possible to allow (full) SSH/SFTP access to the Ubuntu Touch device via `android-gadget-service enable ssh`. This is described in detail at [2].

As an Ubuntu pone user
I want to be able to activate the (unrestricted) access via SSH to my mobile device via an option in the File Manager's drop-down menu (e.g. next to the "Enable full access" option) or on the Places screen somewhere,
which will also tell me the IP address and user name to connect with (for SFTP, SCP, SSH)
and may show a warning or similar in the system indicator area making it clear that access from outside is possible,
so that I have a natural, non-esotheric, FTP-like way of transferring data to my device.

[1] https://uappexplorer.com/app/wifitransfer.sil
[2] http://askubuntu.com/questions/601910/ssh-ubuntu-touch/653595#653595

Side Note
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I've wanted to add this feature request to the Transfer indicator project, but there is no bug tracker configured there.
https://launchpad.net/indicator-transfer

References
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Discussion on the mailing list, "How add custom item to system indicator (network)"
- https://lists.launchpad.net/ubuntu-phone/msg18266.html