* Use Boost pointer container for sheet hierarchy object.
* Add test for modification in sheet hierarchy list object.
* Add method to clear modification flag in sheet hierarchy list object.
* Improve Doxygen comments for sheet hierarchy list object.
* Remove redundant definitions in prototypes header file.
Eeschema: cleaning code. Start work to enhance annotation algorithm.
Already added an option to annotate using sheet number (sheet 1 uses numbers 100 to 199, sheet 2 uses 200 to 299..).
Works fine if deleting existing annotation, but needs work to be used when the existing annotation is kept .