added preferences.ui (gtkbuilder file created with glade)
move preferences.ui to /share/en-linuxclipper directory
Added notebook select feature to enapi class. Related #4
Connect indicator menu and preferences window
new data directory layout and setup.py
don't show preferences window on taskbar
fix NoneType error on ENAPI.get_notebook_list()
Added DIR_PREFIX constant to common.py
Show preferences window on center of screen
Add Preferences class, Related #3 #4