Files app: Implement temporary elevation of privileges (medium)

[jeremypw]

Feature improvement: Rating - medium

At present, in order to carry out administrator tasks with Files you have to open a new window as Administrator.
It would be convenient for Files to temporarily elevate privileges (after authentication) if the user requests an operation requiring administrator privileges such as navigating to a location with no read rights for normal users or carrying out file operations on files without the required permissions for the normal user.

See issue elementary/files#165

Goals

• Allow a normal user to carry out an operation requiring root privileges after providing an administrator password.

Stretch goals

• Allow further admin operations without re-enry of password for the remainder of the session (with a time-out) in the tab being used.
• Visually distinguish tabs that are operating in administrator mode

Skills

• Familiarity with object-oriented programming languages, specifically Vala and the widget toolkit Gtk. It is highly recommended to complete our developer guide before taking on this project
• Good knowledge of Linux security management.