Only attempt to close our peakfile '_fd' if we have a valid handle

(yesterday's fix was needed in a 2nd place)
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
add SVAModifier to ArdourCanvas color code
basic code to use SVModifier plus one definition in dark.colors
fix XML syntax error
fix up SVAModifier constructors
add missing UIConfiguration::modifier()
Remove (no longer needed) header files from our MSVC project (gtk2_ardour)
Add a newly introduced header file to our MSVC project (libpbd)
fix meter color definitions (extra 0x in every one)
more fixes to SVAModifier constructor(s)
Add UIConfiguration::color_mod (colorname, modifiername) to get a modified color
fix incorrect item name for color