Add ag_house_system_id_to_nick() and ag_house_system_nick_to_id()

This is to convert between house system ID (required for chart generating)
and GEnum nick (required for saving).
ag_db_save_identical can now compare based only on chart-related data

This way the chart won’t get recalculated on a name change.
Fix tab_edit widget’s name in .ui file
Rename recalculate_chart() to ag_window_recalculate_chart()

This is simply to follow naming conventions
Refactor ag_window_recalculate_chart() to recalc if edit tab has changed
Merge pull request #44 from gergelypolonkai/recalc-if-needed

Recalculate chart only if needed