fixed merge conflict
merge conflict resolution (text colors are no longer specified explicitly)
move ScopedFileDescriptor into libpbd and use it
initial pass to replace all UIConfiguration::get_XXXXXX() calls with UIConfiguration::color(name).

IMPORTANT: names have not been downcased and spaced yet, so many colors are not found
remove base color concept from UIConfiguration and ThemeManager.

What used to be "relative colors" (defined relative to base colors) are now defined
in absolute terms.
remove now-unused headers
new version of dark.colors
fix up requested color names everywhere.

Thank <deity> for emacs ... space-uncamelcase-word-at-point FTW