Make enumeration counts constant expressions

[ZXShady]

This change allows now having these variables as array sizes that are not Variable Length Arrays like in structs for example.

struct {
   sfKeycode keys[sfKeyCount]; // didn't compile since it was not constant expression
} global_table;

int main(void) {
   return 0;
}

To note however that this changes the type from unsigned int to an implementation defined type which usually int which is signed and not unsigned. and also disallows taking the address of the constant since it is mo longer a variable.

the other is using a macro which I don't like, but it can allow us to change the type to an unsigned type if we really wanted

#define sfKeyCount ((unsigned int)sfKeyPause + 1)

but I prefer CSFML to be macro free if possible so I choose the enum approach.

[eXpl0it3r]

Doesn't that still need to be exported?

[ZXShady]

enums do not provide functionality I don't think they need to be exported and as I see no enums in CSFML are exported

[ChrisThrasher]

For the sake of consistency could you make this change to all similar constants? I think it's just sfMouseButtonCount and sfSensorCount that need this treatment.