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.