Add configuration option/screen for zoom

[kesselb]

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Is your feature request related to a problem? Please describe.

Follow-up for / related to: https://github.com/nextcloud/talk-desktop/issues/74

You can change the zoom level with Ctrl +/- or by using the menu.

A nice enhancement would be to show an "accessibility" option in the settings to configure the zoom like Slack does.

Describe the solution you'd like

I would like a configuration option to adjust the zoom level for the application.

Describe alternatives you've considered

Additional context