Add support for in-app tabs

[stellarpower]

Problem

• Nextcloud is great! As such, I have multiple accounts on my self-hosted device (to delineate different parts of my life - work, personal, etc.) and put various different sorts of content on there (my music, documents, shared scripts, etc.)
• For a long time, I would have liked a quick way to navigate between different accounts, do I don't have to use the back button over and over to make a quick switch between my own server and my hosted accounts.
• However, if I also have several different tasks on the go, it would probably solve both problems to offer a tabbed form of "browsing" in the mobile app(s).

Solution

A top, bottom, or side (possibly configurable) tab stack that presents different pages (i.e. directories, or Android activities if e.g. editing a text file, performing some interaction with a file from inside the app). As these are pretty established UX components and concepts, I don't think I need to expand with any details and instead leave it open to anyone closer to the development team to discuss what would and wouldn't be a good idea with more precision.

Alternative options

• If screen real estate is an issue, perhaps using a circle in the top-right, similar to how the account switcher is displayed, from which the user can access different open pages, would be an option. A similar concept is now employed by the Android version of popular web browsers.
• It would be more thorough to place any views/activities opened within the app inside a tab page container, such that all activities can be scrolled through. If I am uploading some important documents for example, and something comes to mind and I want to add this to a Todo list on the inbuilt markdown editor, it would be convenient for both of these to be able to present themselves inside tabbed views, in addition to the basic display of files in the current directory, so I do not need to lose my flow of working when I am caught in the middle of something.
• However, if this would require a more complex fix initially, simply presenting directory views inside tabs, that would lose focus when interacting with a file took over in a modal fashion, would at least be a good start

Additional context

None relevant as far as I can tell.

Thanks everyone :)

[Yiannis128]

This would be nice, I was going to start an issue about having a Music Player Tab that basically mirrors the music app UI. But this would basically cover that.