Flags cleanup.

Remove extraneous use of FLAG0 from tracks cleaner (it was checked but
never set).

Fix issue in expand connections where two parts of it couldn't agree
on the same flag (BUSY vs SKIP_STRUCT), and where the second part was
clearing the flag instead of setting it.

Remove obsolete HIGHLIGHT infrastructure (we now use selection).

Remove extraneous use of BUSY flag in several places (it was never
set).

common/render_settings.cpp

@@ -33,7 +33,6 @@ RENDER_SETTINGS::RENDER_SETTINGS() :
     // Set the default initial values
     m_highlightFactor    = 0.5f;
     m_selectFactor       = 0.5f;
-    m_highlightItems     = false;
     m_highlightEnabled   = false;
     m_hiContrastEnabled  = false;
     m_hiContrastFactor   = 0.2f; //TODO: Make this user-configurable