Load helpers at View construction.

Loading helpers earlier in View's lifecycle allows for the removal of
many duplicated code segments and a now useless property. It slightly
modifies how View behaves in a test case, but that issue is easily
remedied by calling loadHelpers() a second time.

This primarily fixes issues where helpers may not be loaded in View
subclasses if they override any of View's methods. This is particularly
problematic when aliased helpers are involved.

Refs #4030
Don't load Helpers in dataviews when _serialize is set.

Loading helpers when they aren't going to be used is wasteful.
Merge pull request #1570 from cakephp/issue-4030

2.4 - Load helpers at View construction.