Expose "keycodes_per_modifier" property getter

[andy-zetier]

The GetModifierMapping request returns a list of keycodes. Client code needs to iterate this list in chunks of keycodes_per_modifier to visit all keycodes for each of the 8 modifiers (Shift, Lock, Control, Mod1, Mod2, Mod3, Mod4, Mod5).

The preexisting whitelist of properties/fields that should not be hidden has been extended to include this field.

Fixes #271

The generated code diff looks like this:

diff main/xinput.rs fix/xinput.rs
21933c21933
<     fn keycodes_per_modifier(&self) -> u8 {
---
>     pub fn keycodes_per_modifier(&self) -> u8 {
diff main/xproto.rs fix/xproto.rs
26678c26678
<     fn keycodes_per_modifier(&self) -> u8 {
---
>     pub fn keycodes_per_modifier(&self) -> u8 {