Code beautification in ag-chart.h
Add TODO items
Add note to example chart
Move away from popover

GtkPopover cannot display accels, which is not ideal for the gear/view
menu. Switching to normal menus instead.
Add missing (but actually unnecessary) gtk.h include
Annotated AgDb functions
Merge branch 'master' into chart-db

Conflicts:
        src/ag-chart.h
        src/ag-window.c

Conflicts fixed without problems.