kvui: abstract away client tab additions

[Silvris]

What is this fixing or adding?

Intending for this to get in before #3934, this should allow clients to be cross-compatible between the two client versions. The main (currently-only) failure point for current clients on the KivyMD branch is the swap of GameManager.tabs and what types are accepted its children. As such, this abstracts away this behavior into GameManager itself such that clients need only call the function and provide the content for the tab.

How was this tested?

Loaded up Starcraft 2's client, ensured that tabs worked correctly.

If this makes graphical changes, please attach screenshots.

[qwint]

purely a code review, everything looks good and this will help immensely when balancing apworlds (and PRs) to work for current kivy and the kivymd branch (and will be so nice to clean up code even without that)

[Silvris]

@Ziktofel @FlySniper

[FlySniper]

I ran a brief test where I verified the visual information in the client and the information in the communication files. I also sent a victory file and a location file. Seems to be working.