localize paginator counter output and CakeNumber::toPercentage

Currently L10n is still an issue throughout the core.
There are also different approachs.
One - using setlocale(LC_NUMERIC) I don't like as it creates even more issues and complicates things. Mainly because all the internal output is wrong then (it should stay in the normal numeric format).

As clean approach I recommend using two Configure values, `L10n.decimals` and `L10n.thousands`.
Those will mainly be used for any user output regarding localized numberic values then.

Resolves:
- https://cakephp.lighthouseapp.com/projects/42648-cakephp/tickets/2927-number_format-in-paginationhelpercounter
- https://cakephp.lighthouseapp.com/projects/42648/tickets/2763-cakenumbertopercentage-should-allow-separator-localization