Qt::Application resets numeric locale to "C"

[f-fr]

This is necessary (and adviced by Qt) because Qt sets the locale to
the system default when creating an application object. However,
Crystal functions like to_f depend on the locale being set to "C" by
default.

Because the locale is Unix-specific the reset is done by adding an
additional C helper function, which is called from
Qt::Application#initialize.

For instance, the following example would raise an error with my system locale (de_DE):

require "qt5"

"4.2".to_f   # works
qapp = Qt::Application.new
"4.2".to_f   # fails because the locale has been changed such the decimal separator is "," now and not "."

Because the constant LC_NUMERIC is system dependent (although it's probably always 1 nowadays, but you can't be sure), we add a new C function (instead of just a lib-binding to setlocale from the Crystal side). This makes everything a little bit more complicated because we need to make sure that bindgen finds the new function and generates a binding for it.

[docelic]

Heya,
The patch looks OK.

Is your intention for the patch to have this happen automatically instead of users having to remember to reset the locale?

[f-fr]

Exactly. The main reason is that it is platform specific and there is no standard way to set the local from Crystal (that I'm aware of). Furthermore Crystal itself seems to use the "C" locale by default and probably many libraries expect it to behave accordingly. For instance, `to_f` raising an error on a seemingly valid floating-point number caught me by surprise. IMHO just using Qt should not change the behaviour of any other function and the only sane way seems to be to reset the locale to "C". A better solution might be to query the original locale before the call to the constructor and restore that one afterwards. However, as long as there is no way to change the locale in the Crystal stdlib, this seems pointless to me.

[docelic]

Would you mind also updating the README (maybe somewhere near the bottom?) to mention this?
If we include it, we should also document it since it is a non-standard addition.

If you don't want to bother with the README, no worries, I can do it a little bit later when I get a chance.

Thanks!

[docelic]

Wonderful, thanks!