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). It also breaks lots of normal echo/output calls where there is also normal non-localized numbers expected.

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

Long term I can see those values go into other places where localization should use those config values:
- defaults for format/formatDelta (non currency, though)
- toReadableSize