Moved to gtk_application_set_accels_for_action() API

... and with that, to a GTK+ 3.12 requirement.
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
Make ag_app_buttoned_dialog()'s first parameter nullable
Generalize chart loading functions

* ag_app_import_chart() became ag_app_import_file(), and has the potential
  to load any file type
* ag_app_import_cb() so it can open multiple file types
* ag_chart_load_from_file() is now called ag_chart_load_from_agc()
Define gdouble endianness conversion macros

GLib doesn’t have these for some reason. These are protected in case a
later version of GLib defines them, too.
Create .hor filter for Placidus files
Add Placidus import to the app menu
Import Placidus charts
Merge pull request #45 from gergelypolonkai/hor-import

Import Placidus 4.0 saved charts
Removed graphics-help-from-irc.txt

It was never really needed
Implement chart deletion from database
Add selection mode button and selection toolbar
Implement view-selection-changed signal handler
Delete selected charts
Merge pull request #48 from gergelypolonkai/selection-mode

Implement selection mode and chart deletion