Android: Support Multiple Tabs/Sessions

[AntumDeluge]

💡 Describe your idea

The Android client should support multiple simultaneous tabs/sessions, just like can be done in a normal web browser & how with the classic client multiple instances could be played on at the same time. It might or might not be a good idea to limit the number of tabs though, I haven't decided yet.

I have already added some preliminary work to support this. It should be possible to create multiple instances of ClientView. But we need to add support to the interface for adding, removing, & switching between them.

🔬 Motivation

Traditionally players have been allowed to play with multiple characters simultaneously. The Android client is the only interface that doesn't currently support this (inherently).