sl0ppty: `set-name target="pane"`, so a script can overrule a program's title

A pane's name already outranks the title its program sets -- `pane_title` prefers
the name while there is one -- and that is the whole remedy for a program
announcing something it stopped meaning: an agent still spinning the summary of a
task it finished, a build script that titled itself once. The remedy was reachable
by double-clicking the title on somebody's screen, and from a layout file, but not
from the socket, while `set-purpose` next to it has taken `target:"pane"|"tab"` all
along.

`target` defaults to `tab`, which is what this verb has always meant. Two verbs
whose defaults differ is a smaller wrong than a verb that quietly changes its
subject under scripts already written against it. The refusals say `no such pane`
or `no such tab`, so a mistyped target shows up in the reply rather than as a label
that did not move.

The gesture now calls the same function, so the two ways in cannot disagree about
where a pane's name lives -- there is a test that a name set over the socket is the
name the double-click editor offers to edit.

Verified: eight checks in `tests/test_pane_title.py`. A pane named while its
program keeps setting OSC 2 titles keeps the name; clearing it hands the label back
to what the program calls itself *now*; `id: 0` is the focused pane and leaves the
other one alone; `set-name` with no target still renames the tab and leaves the
pane's title alone.

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