cmIDEOptions: Add an AppendFlag overload to append multiple values
cmIDEOptions: Add HasFlag method to test if a flag is set
VS: Refactor include directory slash conversion

Convert each include directory to windows slashes once instead of
repeating it each time they are written out to the project file.
VS: Refactor include directory generation in .vcxproj files

Use the cmVisualStudioGeneratorOptions flag map to add the
AdditionalIncludeDirectories element to the project file.
Move appending of %(AdditionalIncludeDirectories) to the
locations that populate the flag vectors instead of where
they are written out.
Merge topic 'vs-refactor-includes' into next

5fcb42ab VS: Refactor include directory generation in .vcxproj files
8e026aa7 VS: Refactor include directory slash conversion
9633d112 cmIDEOptions: Add HasFlag method to test if a flag is set
84100102 cmIDEOptions: Add an AppendFlag overload to append multiple values