Cannot type numbers using keypad in TextBox/TextField

[gbaudic]

While trying to solve #4 , I noticed another issue regarding text input: I cannot type numbers using the keypad.
A quick investigation shows that in the Key class, the codes corresponding to the key pad are SDLK_KP_x instead of SDLK_x (with x a number). Unfortunately, according to the SDL docs, SDLK_KP_x has a corresponding integer value way higher than the corresponding ASCII character. This prevents proper insertion of the character in the widget classes.