Convert AgDbChartSave a boxed type
Add AgChartRenderer class

It is a subclass of GdkCellRendererPixbuf specialised in displaying
Astrognome chart previews.
Add AgIconView class

It is a subclass of GtkIconView specialised in displayind Astrognome
chart previews as icons.
Replace GdMainView with AgIconView in the main window
Remove libgd from the dependencies
Code beautification in AgChartRenderer
Remove unnecessary debug lines from AgChartRenderer
Fix AgWindow’s selection mode cancel button
Merge pull request #89 from gergelypolonkai/gtk-icon-view

Move chart list from GdMainView to GtkIconView