Rewrite the add account wizard in QML

[claucambra]

Follow-up from #3896

In line with similar re-writes, it would be a good idea to re-write the add-account wizard in QML for the following benefits:

1. Appearance will be more in line with the tray window and new sharing system
2. Far easier to customise and re-design than with QWidgets
3. Easier integration with the rest of the desktop UI components (i.e. if we want to re-use parts from the tray window or this new component, or place it within the tray window, or anything else)
4. Significantly easier to change and maintain in the future

@tobiasKaminsky @jancborchardt

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Feature description

Tell us how the feature should work

[tobiasKaminsky]

As of https://github.com/nextcloud/desktop/issues/4896 this then would be the tasks:

• Rewrite to QML as we do with other components so we can do proper layouting
• Then we can also e.g. make the whole background blue and the icons and text white, like e.g. on the Android setup screen