Fix select all delay on "select all" checkbox

[matberry]

This fixes #1996

Select all checkbox now works as intended when using setDelaySelectAllEnabled()

Open to criticism since it's my first PR here !

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[lrljoe]

Looks sensible at face value, will need to do some validation before I fire the tests and merge it in though!

[lrljoe]

I've been making some other adjustments to the JS, but am aware/conscious of this PR, so I'll look to incorporate it once I've finished the other adjustments.
Should be at the weekend.