Refactor frontend to use QML / QtQuick.

[christianparpart]

Motivation

• More future-proof / forward compatible Qt API usage
• Hopefully easier to create new UI elements using QML / QtQuick rather than QtWidgets.
• Basinc new ideas on top of this PR, specifically: Tabs and later on split views.
• Having a unified look'n'feel of UI elements. In fact, the scrollbar currently doesn't look nice on all systems, e.g. @whisperity seems to have the default style while I have the awesome style on 0.3.2. With moving the QML/QtQuick it seems possible to have the same look on all platforms and desktops.

This PR "simply" refactors the frontend and ports it from QtWidgets (QOpenGLWidget) to QML / QtQuick. I try not to add any new feature or anything but get it on feature parity.

Checklist

• port to QQuickItem for the Widget
• expose TerminalSession to QML
• ensure functional scrollbar
• ensure window title updates
• use implicitWidth / implicitHeight rather than width / height to declare widget component size
• system beep working
• BUG: currently app termination is assert()ing
• ensure blurred background is working
• UI dialogs working, such as permission request
• notifications-OSC working (Requires Qt 5.12 or higher)
• IME working
• [PLATFORM] ensure proper rendering on Wayland
• [PLATFORM] Ensure it's running on Windows
• scrollbar (move the current JS impl to C++ by introducing a ScrollBar.vertical property (similar to QtQuick's))
  • hide_in_alt_screen:
• platforms (ox/windows/linux): ensure creating new windows in-process and out-of-process is working
  • Windows
  • Linux
• resize
• background image
  • background is loaded from the path
  • opacity of the background image
  • proper layer positions
• opacity
• clean up
  Closes Migrate frontend from Qt Widgets to Qt QML API #956.

[uspasojevic96]

Also add to TODO to not break the functionality on macOS where a single process can have multiple windows instead of spawning entirely new process