`Ui::Animations::Simple`'s api is incomplete. It provides the
`.animating()` method for users to tell if the animation is active, but
it is not sufficient to tell the state when the same animation is used
for both fading in and fading out.

The `.fadingOut()` method is intended to tell these transitions apart.

Intended for telegramdesktop/tdesktop#28811

Previously, `Window::Notifications::Default::internal::Widget` was using
`Ui::Animations::Basic`'s `.animating()` method to tell if a
notification was in the showing state. That is incorrect: a notification
may be in the animating state whether it is fading out (i.e., not
showing) or fading in (i.e., to be shown).

This commit fixes it by using the `Ui::Animations::Basic::fadingOut`
method for telling the notification state instead.

Fixes telegramdesktop#28811.

`Ui::Animations::Simple`'s api is incomplete. It provides the
`.animating()` method for users to tell if the animation is active, but
it is not sufficient to tell the state when the same animation is used
for both fading in and fading out.

The `.fadingOut()` method is intended to tell these transitions apart.

Intended for telegramdesktop/tdesktop#28811

Previously, `Window::Notifications::Default::internal::Widget` was using
`Ui::Animations::Basic`'s `.animating()` method to tell if a
notification was in the showing state. That is incorrect: a notification
may be in the animating state whether it is fading out (i.e., not
showing) or fading in (i.e., to be shown).

This commit fixes it by using the `Ui::Animations::Basic::fadingOut`
method for telling the notification state instead.

Fixes telegramdesktop#28811.

…ently

Previously, `Window::Notifications::Default::Manager` would not start
hiding notifications that are fading in when other notifications should
hide. This would lead to some notifications never hiding, e.g., when the
cursor passes through the notification too quickly and there was not
enough time for the notification to fade in completely.

Fixes telegramdesktop#28811.

…ently

Previously, `Window::Notifications::Default::Manager` would not start
hiding notifications that are fading in when other notifications should
hide. This would lead to some notifications never hiding, e.g., when the
cursor passes through the notification too quickly and there was not
enough time for the notification to fade in completely.

Also renamed `Widget::isShowing` -> `Widget::isFadingIn` for clarity.

Fixes telegramdesktop#28811.