Summary:
This is a preparation step to support idle_inhibit_unstable_v1 protocol.
As in Plasma powermanagement, screen locking, dpms, etc. is not
controlled by the wayland compositor but by external components through
the IdleTimeout interface the compositor needs a way to inhibit the idle
timeouts. So once idle_inhibit_unstable_v1 is implemented the compositor
can hook this up to the inhibit API in IdleInterface and thus inhibit
powermanagement, etc. as requested by the idle_inhibit_unstable_v1
protocol.

The added API is straight forward:
 * inhibit: inhibits idle timeouts
 * uninhibit: uninhibits again
 * inhibit and uninhibit must be called in pairs, so twice inhibit,
means uninhibit must be called twice
 * isInhibited: whether it's inhibited
 * and a signal that it changed

The signal is mostly used internally to stop the timers.

Test Plan: Test case extended

Reviewers: #frameworks, #kwin, #plasma_on_wayland

Subscribers: plasma-devel

Tags: #plasma_on_wayland, #frameworks

Differential Revision: https://phabricator.kde.org/D8383