Added const qualifier to decimalPoint, groupSeparator and groupSeparatorEnabled

Constants are stored in a C locale way and converted to the current locale
settings before being passed to the KNumber constructor.
Unfortunately, the tooltip doesn't show the stored constant in the active
locale as no locale information is stored with the number so that stored constants
between KCalc instances with different locales could be converted on startup.

In D22282#491136, @cfeck wrote:

If the constants are stored in localized format, does changing between locales that swap the decimal separators cause issues?

Another possibility would be to add the removal of the group separators to the KNumber constructor as decimal point conversion is already done there.
I wrote it like this because it is the most non-invasive way and stored constants seem to be the only part affected by it.

Checks if group separators are enabled

new version at D22282