Support showing several views at the same time in the same tab

This would be perfect for a large monitor setup, as one can focus on creating separated views that make sense on their own, and that work fine by switching between them when using a small monitor, but if the user happens to have a large monitor with a good resolution (e.g.: 4K) he can just switch to a grid-mode with many views open at the same time.

This idea came after a talk with @ES-Alexander about supporting multiple tabs.

No files, tests, or entry points are named. Start by locating the existing view and tab-switching implementation, then determine how grid-mode and multiple simultaneous views should interact. Done means several independent views can be displayed together in one tab while existing small-monitor switching continues to work.

Written by the indexing model from the issue text.