Move common keyboard types to iced_core

Also expose them in `iced` through `iced_native` and `iced_web`.

core/src/lib.rs

@@ -14,6 +14,7 @@
 #![deny(unused_results)]
 #![forbid(unsafe_code)]
 #![forbid(rust_2018_idioms)]
+pub mod keyboard;
 
 mod align;
 mod background;