Add a preference to zoom only the focused terminal

[dbolser]

What

Adds a preference, Appearance → "Zoom in/out affects only the focused terminal", so
each terminal can keep its own zoom level. It is off by default, so nothing changes
for existing users.

This addresses the first point of #1568 ("These font size changes apply to all the tabs,
not just the current one").

Terminal already had increase_font_size() / decrease_font_size() and a per-terminal
font_scale index for exactly this, but nothing called them — the accelerators went
straight to the configured font size instead. The preference wires them up.

Behaviour

	zoom in/out affects
preference off (default)	every terminal, by changing the configured font size, so it persists across restarts — as today
preference on	the focused terminal only, as a scale on top of the configured font size

When zooming every terminal, the new size is now taken from the size the focused
terminal is showing, so terminals that had been zoomed individually snap to it instead of
keeping their offsets. Without that, switching the preference off left terminals
permanently different sizes.

Fixes noticed along the way

Both in the zoom-everything path:

• The change was applied twice, once as a font size and once as a Vte font scale
  (set_font_scale(new_size / (new_size - 1))). A step was larger than intended, and
  zoom out did not undo zoom in — 10 → 13 → 10 came back as 10 → 13 → 11.
• Zooming out repeatedly walked the font size down to 0. It now stops at 1.

Dropping the Vte font scale means the zoom-everything path no longer works by
multiplying the displayed font, so when use-default-font is set (the schema default)
— where fstyle_changed ignores the configured size and follows the desktop font — it
now zooms every terminal by the same font_scale level instead. Same visible result,
one mechanism.

Testing

Manually, in a nested X server, for both use-default-font values:

• preference off: all terminals zoom together; the configured size round-trips exactly
  (10 → 13 → 10)
• preference on: only the focused terminal changes; the configured size is untouched;
  splits and other tabs keep their own levels; new tabs open at the configured size
• three tabs at three different levels, then preference off and one zoom in: all three
  snap to the focused tab's size
• clamped at both ends without raising

black, flake8 and the glade/gschema parse are clean; the release note is in
releasenotes/notes/.