macOS: Remove is_key_down from ViewState (#1489)

Fixes #1488.

`is_key_down` was only set to true when `insertText` was called.
Therefore `is_key_down` was actually meant to store whether the most
recently pressed key generated an `insertText` event, at which, winit
produces a `ReceivedCharacter`. The issue is that `insertText` is *not*
called for "key repeat", but winit wants to send `ReceivedCharacter`
events for "key repeat" too. To solve this, the `is_key_down` variable
was then checked in the `key_down` function to determine whether it was
valid to produce repeated `ReceivedCharacter` events during "key
repeat". However this check is not needed since `ReceivedCharacter` must
always be called if the key event has a non-empty `characters` property.

Furthermore `is_key_down` didn't actually store whether the previously
pressed character had an `insertText` event, because it was incorrectly
set to false on every "key up". This meant that if two keys were pressed
consecutively and then the first was released, then `is_key_down` was
set to false even if the most recent keypress did actually produce an
`insertText`.

Update changelog.

CHANGELOG.md

@@ -5,6 +5,7 @@
 - **Breaking:** On Web, remove the `stdweb` backend.
 - Added `Window::focus_window`to bring the window to the front and set input focus.
 - On Wayland and X11, implement `is_maximized` method on `Window`.
+- On macOS, fix issue where `ReceivedCharacter` was not being emitted during some key repeat events.
 
 # 0.25.0 (2021-05-15)