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
Fixed typo in comments
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
Fixing submodules
Merge tag '2.4.0' of git://github.com/cakephp/cakephp

CakePHP 2.4.0

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