osc 5577: `busy`, and a sidebar that shows it (busy_mark)

A status is a line of text, and a line of text cannot say whether it is still
true. "building 3/7" reads exactly the same while it builds and ten minutes
after it stopped, which is the one thing somebody reading it from another tab
wants to know -- and the only workaround available to a program was to write
the answer into the words and keep rewriting them.

So `busy`:

    printf '\033]5577;1;status;make test\033\\'
    printf '\033]5577;1;busy;1\033\\'
    ...
    printf '\033]5577;1;busy;0\033\\'

A flag of its own rather than a field of `status`, because a status is the
*whole* payload after the verb -- `status;a;b;c` has always been the text
"a;b;c" -- so anything added to that line would change what every existing
sender means, including the five pi extensions D1 promises byte compatibility
to. Orthogonal, so a program can leave one line up and toggle the other;
anything but 0/false/no/off is on; `clear` takes both; and a pane whose program
has gone is never busy, whatever it last claimed, because a spinner turning
under a dead tab is a lie you keep looking at.

In a sidebar the row turns `tab_status_busy` -- the theme's accent, derived
like the other status colours, so no theme needs a line -- and takes a spinner
in the column `tab_bar_padding` was already holding blank, so nothing reflows
when a program starts or stops working. `busy_mark` is the frames, separated by
spaces: one frame is a static mark and costs nothing, several make it turn, and
only then does the session keep a frame clock -- via the same `animating` flag
an animated shader uses, so the cost lands only while such a row is actually
composed, and `""` drops the mark entirely and leaves the colour to say it.
The frame is a pure function of the clock, like every other pulse here, so two
clients attached to one session show the same one.

`panes` now reports `status` and `busy`, which is the scriptable half of the
same fact: what a pane says about itself, without reading its screen.

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