Allow to set a standard account that shall be displayed in the main window when clicking on the tray icon if there is more than one accounts

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## Feature request

**Which Nextcloud Version are you currently using:** (see administration page)
3.16.6.65530

**Is your feature request related to a problem? Please describe.**
If there is more than one account:
I want a certain account to be seen in the main window after a Windows start when clicking on the tray icon.

**Describe the solution you'd like**
Allow to set a standard account that after a Windows start shall be displayed in the main window when clicking on the tray icon.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.