Core: Arrays like [42] should fail .isNumeric()

Fixes #14179
(cherry picked from commit 10efa1f5b44046aab6bcc8423322a41923faa290)
Ajax: .load() should trim its selector

Fixes #14773
(cherry picked from commit 3a68c114e3a30af86dc261c9d62ce2ae4144d420)

Conflicts:
	src/ajax/load.js
Event: Allow triggerHandler(beforeunload)

Fixes #14791
(cherry picked from commit 06adf7c95d93507908181995a9ea15fe2729595a)

Conflicts:
	src/event.js
CSS: Return values should be numbers

Fixes #14792
(cherry picked from commit f4b37d89820535d6c7503925aa8872645681a865)
Core: Do not run window.onready when ready

Fixes #14802
(cherry picked from commit 2df1aad6a1c9376c2a477eba26ee992113ed1c23)
Ajax: Use two-arg form of .slice() for old IE

Followup to a4b9bc57, #14773