Change preferences sub-dialogs to be modal

This solves the issue where the parent dialog is closed and then
the child dialog is used.

This is however only a partial fix:

- Many other dialogs throughout the codebase do not currently have
  parent windows and need to be refactored.

- Not all window managers respect modal so users can still trigger
  bugs. We can be more defensive against this but it requires more
  refactoring.

Closes #2686

src/common/fe.h

@@ -141,6 +141,7 @@ void fe_get_int (char *prompt, int def, void *callback, void *ud);
 #define FRF_NOASKOVERWRITE 32	/* don't ask to overwrite existing files */
 #define FRF_EXTENSIONS 64		/* specify file extensions to be displayed */
 #define FRF_MIMETYPES 128		/* specify file mimetypes to be displayed */
+#define FRF_MODAL 256           /* dialog should be modal to parent */
 void fe_get_file (const char *title, char *initial,
 				 void (*callback) (void *userdata, char *file), void *userdata,
 				 int flags);