Added support for localized (and parameterized) numeric format

Formatting numeric values sometimes involves decimals separator and thousands separator.
A wide variety of formats exists in the world (see http://en.wikipedia.org/wiki/Thousands_separator#Digit_grouping), but currently Joomla only uses the American format, where the decimals separator is a mark and the thousands separator is a comma (2,000.5 MB). For example, European format is the opposite (2.000,5 MB).
Even worse, this format is hardcoded into the code so that it can't be easily changed.

This contribute removes the constants "." and "," used along the Joomla code, and introduces the use of DECIMALS_SEPARATOR and THOUSANDS_SEPARATOR language strings, which will be localized by the language Teams.