Added libgd as a submodule, until GTK+ 3.10 comes out

libgd provides some neat widgets, like a header similar to
GTK+ 3.10's GtkHeaderBar, which comes in handy when generating the
window headers.
Created small TODO file
Started porting to GApplication
Created resource from astrognome.ui
Added libgd as a compile time dependency

It is now used temporarily to create a header bar in the first window
Created the wrapper application AgApp

AgApp is a subclass of GtkApplication, and is used to wrap around the
whole Astrognome application
Removed About dialog from Glade resource
Created the AgWindow class for the chart window
Now using AgWindow instead of a simple GtkActionWindow
Started implementing tab changing menus