sl0p: keep stacked-layout entries identifiable

Two problems made the stack list unusable for panes nothing had named yet,
which is the common case right after opening them -- and the responsive
collapse walks straight into it, since collapsing is exactly when the list
becomes the only way to pick a pane.

`hide_default_pane_names` no longer applies to stack list entries. In a frame
an empty title is fine: the pane is still visible, sized and positioned, so
you can tell what it is by looking at it. In the stack list the label is the
ENTIRE affordance, so blanking it leaves rows that cannot be identified or
told apart. Entries fall back to the "Pane #N" placeholder even when frames
are configured to hide it.

Entries are also prefixed with their 1-based position, so they stay countable
and addressable even when several share a title or have none. The prefix is
built by one shared helper because the renderer and Tab's column-width
calculation must agree on its width or the list misaligns.

0 files changed, 0 insertions, 0 deletions