…anel

We forgot to handle the "suggestions" and "commitDescription" view names.

Instead of listing all the names of views that can appear in popups though,
let's use the context kind for this, which feels more robust.

This is a change in behavior: previously, clicking outside of the search or
filter prompt would close the prompt, now it no longer does (because search has
a persistent popup kind, but it wasn't listed in the list of view names before).

…at a time

This is how we do it for confirmation with suggestions too, so be consistent. It
will make things easier later in this branch if we only have one context per
"panel" on the stack, even if the panel consists of two views.

Concretely this means:
- only push the message context onto the stack when opening the panel (this
  requires making the description view visible manually; we do the same for
  suggestions)
- when switching between message and description, use ReplaceContext rather than
  PushContext

ResizeConfirmationPanel and resizeConfirmationPanel were identical, get rid of
one of them.

We resize the panel in layout, so there's no need to do that after creation.

The only purpose of this was to scroll the editable text correctly (see
#2146); now that gocui takes care
of that, we no longer need to do this.

It always returned nil, so there's no point in returning an error.

Probably not the most import feature in the world, but when resizing the
terminal window while multiple popup panels were open at the same time, we would
only resize the topmost one.

The main reason for changing this is because it makes the next commit easier to
implement.

When opening a popup panel on top of another one, offset the new one a little
bit down and to the right.