Fixes#3994.
Task creation was failing for some users because Windows Tasks are all
stored in a shared folder, so attempts to use the same task name by
different users were failing, especially if the user was a non-admin
user.
Furthermore, attempts to execute the trigger would fail because Windows
treats keyman.exe as needing elevation in this context (even though it
doesn't, actually), and so I changed the trigger to call `kmshell -s`,
which then does the needful.
Finally, the uninstaller now attempts to remove all the scheduled tasks
in the Keyman folder. It doesn't care if it fails -- it's just cleanup
and doesn't impact the system adversely if the tasks hang around.
