[addons] move the 'check for updates' item handling to the onclick handler

Rather than a fake directory item.  This means addons://check/ isn't called from other spots that are inappropriate (e.g. it isn't the folder that you reside in which then hits an infinite loop as the repo update fires an event to refresh the folder...)

Fixes #15363

Note that this changes the action of this entry - previously it would check, and if updates were found, would show a list thereof.  Now it just checks.

@MartijnKaijser mind checking it does the trick?

@da-anda mind checking the UX with this change?