XDG_APPS_INSTALL_DIR and KStandardDirs

[Alex Merry]

This got a bit lost in the other thread on the kde4support tests...

I want to advocate setting XDG_APPS_INSTALL_DIR to be
$CMAKE_INSTALL_PREFIX/share/applications instead of
$CMAKE_INSTALL_PREFIX/share/applications/kde5.

I think we should be putting our application desktop files directory in
share/applications, not in share/applications/kde5.

The reason this has come up is that KStandardDirs currently contains a
hack for how it compiles the value of xdgdata-apps.  The upshot of this
is that if the directory $CMAKE_INSTALL_PREFIX/share/applications does
not exist, KStandardDirs::resourceDirs("xdgdata-apps") does not contain
that directory, but instead contains XDG_APPS_INSTALL_DIR, which is
$CMAKE_INSTALL_PREFIX/share/applications/kde5.

In normal circumstances this does not happen (because the directory
normally does exist, and if it doesn't it's kind of irrelevant), but it
does cause one of the unit test to fail on Jenkins.  Changing
XDG_APPS_INSTALL_DIR would fix it, but it is not the only way to fix it.
 I just think that XDG_APPS_INSTALL_DIR *should* have a different value.

Alex