Enable custom templates for sketch.cpp and sketch.h

Added come constants for flag indicating default or custom sketch
template and absolute folderpath for custom template files
(ArduinoConst.java)
Added getters and setters for constants
(ArduinoInstancePreferences.java)
Modified ArduinoSketchWizard to handle the templates, save the constants,
build the project and added comments (NewArduinoSketchWizard.java)
Added DiskStream.java to handle files with absolute paths instead of
relative paths
Added SketchSelectionPage.java and SketchTemplatePage.java to hande the
ability to choose between default and custom template files.