feat: Push notifications

[alexjba]

This commit provides QML ready APIs for push notifications.

Available API:

• PushNotifications.status (Unknown, Granted, Denied). Represents the OS permission state
• PushNotifications.token (iOS only TODO: Android). The token is necessary in order to register a device for push notifications. It's usually registered in the backend to provide push notifications regardless if the app is open or not.
• PushNotifications.request(). Request push notifications permissions from the OS
• PushNotifications.requestToken() (iOS only TODO: Android). Request the push notifications token from the OS
• PushNotifications.openSettings(). Open the app settings to update the push notifications permissions. This is necessary once the user has denied the initial OS permission dialog. It cannot be triggered again and the user needs to navigate to settings in order to update the permissions.
• PushNotifications.showNotification(title, message, identifier). Display a local notification
• PushNotifications.clearNotifications(identifier). Clear app notifications

[alexjba]

Android is a little bit special here. It needs some java implementation and as a result the MobileUI lib will also provide a jar. When compiled with FetchContent as we're doing in StatusQ it's easy because all we need to do is to declare the dependency in the app gradle file and the lib will be pulled on app pkg from the maven repo.

[alexjba]

All the gradle stuff is just copied from QT installation. We could do it without it, but it complicates things unnecessarily for the cmake

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[caybro]

LGTM!