Coding style fixups

This commit adjusts the app chooser code to GTK+ coding style:
- line up prototypes
- remove some excess {}
- remove tabs and trailing whitespace
- add docs

tests/testappchooser.c

@@ -81,8 +81,7 @@ prepare_dialog (void)
 
   if (use_file)
     {
-      dialog = gtk_app_chooser_dialog_new (GTK_WINDOW (toplevel),
-                                         0, file);
+      dialog = gtk_app_chooser_dialog_new (GTK_WINDOW (toplevel), 0, file);
     }
   else
     {