* Add is_ and as_ methods to the event enums
Often application code only cares about a small subset of possible
events. These methods make it simpler to write code which checks whether
an event is a particular event type or converts events into the specific
type (returning an Option).
This can help simplify some nested match blocks. E.g.:
```rust
match event {
Event::Key(key) if key.kind == KeyEventKind::Press => { ... }
}
```
becomes:
```rust
if let Some(key) = event.as_key_press() { ... }
```
Similar flexible methods are aded across all the event enums:
- `Event::is_focus_gained()`
- `Event::is_focus_lost()`
- `Event::is_key()`
- `Event::is_mouse()`
- `Event::is_paste()`
- `Event::is_resize()`
- `Event::is_key_press()`
- `Event::as_key_press() -> Option<&KeyEvent>`
- `MouseEventKind::is_*()`
- `MouseButton::is_*()`
- `KeyEventKind::is_*()`
- `KeyEvent::is_press()`
- `KeyEvent::is_release()`
- `KeyEvent::is_repeat()`
- `KeyCode::is_*()`
- `KeyCode::is_function_key(n)`
- `KeyCode::is_char(c)`
- `KeyCode::as_char() -> Option<char>`
- `KeyCode::is_media_key(media)`
- `KeyCode::is_modifier(modifier)`
- add is_key_release() and is_key_repeat() checks
- add as_key_event()
- rename as_key_press() to as_key_press_event()
- add as_key_repeat_event()
- add as_key_release_event()
- add as_mouse_event()
- add as_paste_event()
- more tests
- update event-match and key-display examples
