Move common keyboard types to iced_core

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

native/src/input/keyboard/modifiers_state.rs

@@ -1,15 +0,0 @@
-/// The current state of the keyboard modifiers.
-#[derive(Debug, Clone, Copy, PartialEq, Default)]
-pub struct ModifiersState {
-    /// Whether a shift key is pressed
-    pub shift: bool,
-
-    /// Whether a control key is pressed
-    pub control: bool,
-
-    /// Whether an alt key is pressed
-    pub alt: bool,
-
-    /// Whether a logo key is pressed (e.g. windows key, command key...)
-    pub logo: bool,
-}