Disable idle detection label if not available

If idle detection is not available, disable the label together with the
checkbox. That improves the visibility of the "disabled state".
Fix platforms ifdefs for Qt 5

The Q_WS_ ifdefs are gone in Qt 5.
For OS X, use Q_OS_OSX (Q_OS_MAC also includes ios, which we do not yet
support and where most/all of the ifdefs won't apply.
Merge pull request #164 from frankosterfeld/work/some-fixes

Work/some fixes