Move common keyboard types to iced_core

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

examples/pane_grid/Cargo.toml

@@ -7,4 +7,3 @@ publish = false
 
 [dependencies]
 iced = { path = "../.." }
-iced_native = { path = "../../native" }

examples/pane_grid/src/main.rs

@@ -1,8 +1,8 @@
 use iced::{
-    button, pane_grid, scrollable, Align, Button, Column, Container, Element,
-    HorizontalAlignment, Length, PaneGrid, Sandbox, Scrollable, Settings, Text,
+    button, keyboard, pane_grid, scrollable, Align, Button, Column, Container,
+    Element, HorizontalAlignment, Length, PaneGrid, Sandbox, Scrollable,
+    Settings, Text,
 };
-use iced_native::input::keyboard;
 
 pub fn main() {
     Example::run(Settings::default())