Ubo-gui

- refactor(NotificationWidget): improve layout of the notification widget

- fix(menu): use application's title as the title of the menu if it's provided
- fix(menu): add the title space to the application if it has no title

- feat(Menu): render faded next and previous menu items to induce there are more items in the menu and it can be scrolled
- refactor(core): add colors to list of global constants to avoid hardcoding
- refactor(AnimatedSlider): not using the default look, replaced with the one designed in figma
- refactor(ItemWidget): add `opacity` field
- ci(github): add changelog to the release notes

- fix(regression): handle return value of the action in `ActionItem` when it's callable
- refactor(notification): make the icon smaller and make sure it is rendered on top

- fix(menu): not assume the return value of the action in `ActionItem` is a `Menu` if it is not an `application`

- feat: info action for the `NotificationWidget`