sl0p: optionally hide the 'Pane #N' placeholder title

`ui { pane_frames { hide_default_pane_names true } }`, default false.

A pane now shows no title at all until something actually names it: the user
renaming it (including by double-clicking the title row) or the running
program setting a title. Upstream always shows an auto-generated 'Pane #N'.

Implemented by tracking whether pane_title is still the constructor's
placeholder, NOT by matching the string: a program that legitimately titles
itself 'Pane #3' must not be mistaken for an unnamed pane. set_title clears
the flag, since something then chose that title.

current_title reads the option at render time rather than baking an empty
title in at construction, so it hot-reloads and turning the option back off
restores the placeholder on existing panes.

Display precedence is unchanged and now explicit in the code:
    user-given name > title set by the running program > placeholder

render_terminal_title already fell back to an empty string rather than the
placeholder, so the terminal WINDOW title is unaffected.

Empty titles were already handled by the frame renderer
(render_title_left_side returns None for an empty title), including the
sl0p centred/right alignment path.

Baselines held: zellij-server 1436/30, zellij-utils 436/5.

0 files changed, 0 insertions, 0 deletions