Core: Remove native String#trim usage to save size

Fixes #14794
Core: Arrays like [42] should fail .isNumeric()

Fixes #14179
Ajax: .load() should trim its selector

Fixes #14773
Event: Allow triggerHandler(beforeunload)

Fixes #14791
CSS: Return values should be numbers

Fixes #14792
Core: Do not run window.onready when ready

Fixes #14802
Build: Fix AMD option for custom builds

Fixes #14859
Manipulation: don't use Object.keys for consistency

Fixes #14659
Ajax, Effects: Disable Chrome 31 workarounds
Css: Revert 24e587929f62428e1959b10aace6dc4fd65ab397

The workaround to be able to change !important styles broke the browser
keeping the old CSS value if the new one was rejected. Patching it would
involve a significant perf hit (~33%) so the initial patch needs to be
reverted instead.

Tests by m_gol & gibson042.

Fixes #14836
Closes gh-1532