Make functions static, ensure declarations match headers (#610)
    
    This patch used -Wmissing-declarations warning

    to show functions and symbols that had no declarations, and either:

    

    - Make static

    - Add to header

    

    This helps avoid possability that declarations and functions get out of sync.

    And ensures all source files reference headers correctly.

    

    It also makes sure functions defined with extern "C",

    have this defined in the header. An error found in calligraph.h while writing this patch.

    

    This has been applied to toonzlib, to avoid making very large global changes.

    If accepted, -Wmissing-declarations warning could be added to CMake.