desktop-entry: demote libsfdo error-logging

desktop-entry: demote libsfdo error-logging

...to WLR_INFO to avoid logging issues with .desktop files as errors, for
example:

    [sfdo-desktop] 1:1: Name is unset
    [sfdo-desktop] Failed to load /usr/share/applications/lxqt-panel.desktop
    [sfdo-desktop] 1:1: Exec is unset while DBusActivatable is unset or false
    [sfdo-desktop] Failed to load /usr/share/applications/qemu.desktop

Also make libsfdo debug/info logging depend on environment variable
LABWC_DEBUG_LIBSFDO being set to avoid disproportionately verbose logging
by default for one particular sub-system.

Add an 'ENVIRONMENT VARIABLES' section to labwc(1) to describe the above
as well as other hitherto undocumented env vars with prefix LABWC_DEBUG_.