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