xdg-screensaver should control X11's screensaver in Xfce as fallback

[Simon Steinbeiß]

Problem description
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Expected behaviour:
When playing back videos in fullscreen mode in Xubuntu's, the X11 screensaver should be inhibited. (Xubuntu previously used XScreensaver.)

Behaviour:
The screensaver blanks the screen (after the timeout that the user defined in light-locker-settings, by default 10mins) despite the video playing in fullscreen.

[Test Case]
Play back a video file in Parole with screenblanking enabled (can be checked with "xset q | grep timeout").

[Regression potential]
None apparent, tested in Trusty.

[Other Info]
A little background: this problem became apparent only in Trusty with the switch from XScreensaver to light-locker. As light-locker doesn't handle screenblanking itself but instead relies on X11's builtin screensaver (aka MIT screensaver extension), it needs xdg-utils/screensaver's support for inhibiting the blanking. For some reason, xdg-screensaver doesn't support controlling X11's screensaver extension in Xfce, probably for historical reasons (e.g. most distros with Xfce use XScreensaver).