sidebar: air inside it, the way panes have padding (tab_bar_padding)

`padding` is a pane's: the ring between a frame and the text it holds, in rows
times gap_aspect so a `1` looks square around a block of prose. Handing the
same knob to the sidebar looked tempting and is wrong twice over. The sidebar
is chrome, not a pane, and its width is fixed by tab_bar_width -- so `padding
1` would quietly eat four of a sixteen-column strip (two cells a side, aspect
included) and shorten every tab name, because somebody wanted their terminal
text to breathe. And its two axes are not one idea: the columns are an indent
for labels, the rows are air above a list.

So tab_bar_padding, its own knob in the family the rest of the sidebar's air
already lives in (tab_bar_pad, tab_gap, newtab_pad), taking the same 1/2/4
CSS-order values as `padding` -- one reader for both, so they cannot drift
into meaning different things -- but counted in cells.

The default `0 1` is what the sidebar already drew: that one column is where
every label's leading space has been hard-coded since the sidebar existed, and
moving it into a setting is what makes `tab_bar_padding 0` possible at all --
names flush against the frame, which was not previously a thing anyone could
ask for. The statuses, the `+ new tab` row and the pane count follow the same
indent, since they were each carrying their own copy of that space.

It is air *inside* the plate: a tab's fill still spans the whole row and the
whole row is still the hit, because the row is what you click and a highlight
that stopped short of the frame would read as a highlighted word rather than a
chosen row. The bottom rows are given up when the strip is short of them, like
every other piece of air in here.

tab_bar_pad is the older name for the top component alone. It still works,
still means exactly that, and wins where a config writes both -- it is the
more specific of the two. The dump writes tab_bar_padding and mentions it.

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