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()

src/resources/ui/astrognome.ui

@@ -11,6 +11,7 @@
             <item>
                 <attribute name="label" translatable="yes">Import</attribute>
                 <attribute name="action">app.import</attribute>
+                <attribute name="target">agc</attribute>
                 <attribute name="accel">&lt;Primary&gt;o</attribute>
             </item>
         </section>