Add new Symbolic-High-Contrast and Symbolic-Inverted-High-Contrast
themes, which are automatically-generated high-contrast variants of
the (original) Symbolic theme. The contrast factor is settable in
the makefile, and is currently at 1.5 for both themes.
Rename tools/invert-svg to tools/svg-contrast, which now takes a
contrast-factor argument, and adjusts the input SVG contrast,
instead of just inverting it. Note that we can still use the tool
to invert icons, using a contrast of -1.
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