The Brush / Rect / Fill tool buttons in the per-key color editor now
show icons instead of text labels, with the original labels retained
as tooltips and accessible names.
Icons are recolored at load time to match the active GTK theme's text
foreground (light grey on dark themes, dark on light), by reading the
button's style context and substituting currentColor in the SVG before
loading the pixbuf. GTK's stock symbolic loader is bypassed because it
only recolors fill palette stand-ins and ignores stroke="currentColor",
which is what the source SVGs use.
Adds MIT-licensed Tabler icons under share/solaar/icons/, see
THIRD_PARTY.md.
Ken Sanislo