Use the IEC power symbols for GtkSwitch labels

Use the IEC power symbols for GtkSwitch labels

Unicode 9.0 introduced glyps for the "on" and "off" power states, in the
form of:

 - U+23FD POWER ON SYMBOL, or ⏽
 - U+2B58 HEAVY CIRCLE, or ⭘

With `HEAVY CIRCLE` as "power off symbol" selected to avoid adding yet
another circle to the standard.

Since we moved GtkSwitch to always show glyphs instead of (translatable)
strings, asking the localisation teams to either come up with a suitable
short string to replace the English "ON" and "OFF", or to fall back to
Unicode glyphs, we should ensure we're using the appropriate symbols to
begin with.

See also: gtk!503 for the corresponding gtk-3-24 change.