monitor-manager: Remove duplicate variable definition
monitor-manager: Rename output_id to output_index

The term "output_id" is already used as an opaque ID.
window: Don't treat an output_id of 0 as an invalid ID
monitor-manager: Rename output_id to winsys_id

The output_id is more of an opaque identifier for the monitor, based on
its underlying ID from the windowing system. Since we also use the term
"output_id" for the output's index, rename our use of the opaque cookie
"output_id" to "winsys_id".