GScan deep watchers

[MetRonnie]

https://github.com/cylc/cylc-ui/pull/1269#discussion_r1160035453

Problem

Deep watching workflow tree may have performance hit because we don't need to recompute the filter on every change e.g. job messages

https://github.com/cylc/cylc-ui/blob/b742553a0e809f8e07402deedb0c72c769c10f92/src/components/cylc/gscan/GScan.vue#L273-L277

Proposed Solution

Refactor.

Solution might be to use intermediate computed properties that pull out the relevant fields and watch those instead.