Fix for #3515, functional extension & tests

https://cakephp.lighthouseapp.com/projects/42648-cakephp/tickets/3515

I used (string) because it is faster than strval() and there are no real benefits in this case.
Also enabled assign(), append() and prepend() to take any values convertible to string.

Removed try/catch as discussed with ADmad.
Changed the three exception expecting tests to check for PHPUnit_Framework_Error now.
Extract category strings to separate .po files
Fixed cross-platform issue: LC_* constants have different values on windows
Fixed category constant values: constants in Cake don't use the same values as PHP (Linux)
Fixed typos in text and comments
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
Fixed typo in comments
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.
Removed unused variable
Merge pull request #1563 from mvdriel/fix-i18n-extract

Extract category strings to separate .po files
Merge pull request #1456 'fix-for-3515' into 2.4

Conflicts:
	lib/Cake/Test/Case/View/ViewTest.php

Fixes #3515
Refactor tests to use a data provider.
Update version number to 2.3.10
Merge branch '2.4' making 2.4 into the master branch.

Conflicts:
	lib/Cake/VERSION.txt
Update version number to 2.4.0
Remove JS constant use from tests.
PHPCS fixes.
Updates the Travis button to the master branch.
Added @link to all public functions there have documentation.
And correct a small 'typo'.