iPhone app, round 7: one view switch, Panes only in a session

A

View switch

The view switch now lives only at the bottom right of the Code composer. The header button is gone.
Its icon shows the current view. One tap opens Code, Terminal and Shell, with the current one ticked.
The shell has no composer, so the same button sits at its bottom right, above the keys. You get back to Code or Terminal from there.
One implementation is used in both places. The old header button and its picker sheet are deleted.

B

Panes

The Panes button appears only once a thread has a session. It is not on the Code home or on a new, empty thread.

C

Header now

On the Code home: ☰, the gizzi code logo, and the sessions button.
In a session: ☰, the session name, Panes, and the sessions button.

D

Checked

The build passes. I checked the Code home, a new thread, the Terminal look and Shell in the simulator.
The docs are updated and the link check reports 0 problems. Pushed to #1942.

# iPhone app, round 7: one view switch, Panes only in a session

## View switch
- The view switch now lives only at the bottom right of the Code composer. The header button is gone.
- Its icon shows the current view. One tap opens **Code**, **Terminal** and **Shell**, with the current one ticked.
- The shell has no composer, so the same button sits at its bottom right, above the keys. You get back to Code or Terminal from there.
- One implementation is used in both places. The old header button and its picker sheet are deleted.

## Panes
- The Panes button appears only once a thread has a session. It is not on the Code home or on a new, empty thread.

## Header now
- On the Code home: ☰, the gizzi code logo, and the sessions button.
- In a session: ☰, the session name, Panes, and the sessions button.

## Checked
- The build passes. I checked the Code home, a new thread, the Terminal look and Shell in the simulator.
- The docs are updated and the link check reports 0 problems. Pushed to #1942.