status/a11y: Block 'toggled' signal when manually toggling

status/a11y: Block 'toggled' signal when manually toggling

When the 'text-scaling-factor' key changes and it's set bigger than 1.0,
the corresponding switch is activated in the a11y menu. However, since
commit 946ee936926a0e14fa5f86f62da757bcb581a147, setting the state using
`setToggleState()` also fires the 'toggled' signal, which is I think
correct and intended.

However, that means the GSettings handler for 'text-scaling-factor'
should block that 'toggled' handler when it sets the switch active, as
otherwise the 'text-scaling-factor' is always set to 1.25 when it is
first made bigger than 1.0.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3574>
(cherry picked from commit 1e6c92a083864da9914078990210c1eb38c38cb1)