color: call use_default_colors() in a single location

color: call use_default_colors() in a single location

we call use_default_colors() when parsing colors in rc files only, and
unconditionally when defining the color of the tree element, because the
foreground color of the tree element may be combined dynamically with
the default background color of another element, not explicitely defined
in an rc file. This patch fixes a bug visible with some versions of the
ncurses library: use_default_colors() was used too late, and generated
color leakage effects. (#689)