GScan deep watchers

[MetRonnie]

#1269 (comment)

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

cylc-ui/src/components/cylc/gscan/GScan.vue

Lines 273 to 277 in b742553

	workflows: {
	deep: true,
	immediate: true,
	handler: 'filterWorkflows'
	},

Proposed Solution

Refactor.

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