Support platforms without KDE idle mechanism

This change allows aw-watcher-window-wayland to work even if the KDE
idle mechanism is not present. It does that by attempting to instantiate
a KDE Idle object as normal, but not panicking if that fails, and
instead marking the idle part of the watcher as inactive, and not making
subsequent calls to it.

The background is that Issue #35 [1] reports that sway has switched idle
mechanisms to not use the KDE idle mechanism. Presumably, other window
managers may also fail to support the KDE idle mechanism. Pending full
support for idle-notify-v1, it's still useful to use activity watch on
sway to get focused window titles, so this commit allows the program to
run on sway.

Some changes to Cargo.lock were necessary to compile.

[1] #35