Change foreground color handling for SVG files (bug#77841)

Change foreground color handling for SVG files (bug#77841)

* etc/images/checkbox-mixed.svg:
* etc/images/checked.svg:
* etc/images/conceal.svg:
* etc/images/down.svg:
* etc/images/gnus/gnus-pointer.svg:
* etc/images/left.svg:
* etc/images/outline-close.svg:
* etc/images/outline-open.svg:
* etc/images/radio-checked.svg:
* etc/images/radio-mixed.svg:
* etc/images/radio.svg:
* etc/images/reveal.svg:
* etc/images/right.svg:
* etc/images/symbols/check-mark_16.svg:
* etc/images/symbols/chevron_down_16.svg:
* etc/images/symbols/chevron_left_16.svg:
* etc/images/symbols/chevron_right_16.svg:
* etc/images/symbols/chevron_up_16.svg:
* etc/images/symbols/cross_16.svg:
* etc/images/symbols/cross_circle_16.svg:
* etc/images/symbols/cross_circle_fill_16.svg:
* etc/images/symbols/dot_large_16.svg:
* etc/images/symbols/dot_medium_16.svg:
* etc/images/symbols/dot_small_16.svg:
* etc/images/symbols/heart_16.svg:
* etc/images/symbols/heart_fill_16.svg:
* etc/images/symbols/heart_half_16.svg:
* etc/images/symbols/menu_16.svg:
* etc/images/symbols/minus_16.svg:
* etc/images/symbols/minus_circle_16.svg:
* etc/images/symbols/minus_circle_fill_16.svg:
* etc/images/symbols/plus_16.svg:
* etc/images/symbols/plus_circle_16.svg:
* etc/images/symbols/plus_circle_fill_16.svg:
* etc/images/symbols/star_16.svg:
* etc/images/symbols/star_fill_16.svg:
* etc/images/symbols/star_half_16.svg:
* etc/images/unchecked.svg:
* etc/images/up.svg: Set 'fill' color to 'currentcolor'.
* etc/images/symbols/README: Add explanation of change to instructions.
* src/image.c: Remove setting of the 'fill' color in the default SVG
stylesheet.

(cherry picked from commit 4d145d08da86a05187f99ab6352637241e146547)