InhibitPortal: Implement SessionMonitor

David Edmundsonrequested to merge
work/d_ed/logout_inhibition into master

This is used for applications to inhibit logout.

The design is that plasma-shutdown will call into here to query if log out is possible; this will in turn ask all applications if it is ok to quit.

This makes it slightly different from other portals in that we have non-sandboxed parts calling into here.

This patch also fixes a potential race with power inhibition if a caller closes the request super early.

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