Fix leak in "Open Font" dialog file filter list

Each time the "Open Font" dialog is displayed, `fontforgeexe/openfontdlg.c` :: `FVOpenFont()` fetches a list of file filter types from `StandardFilters()`.  The list 'filts' is later used in setting up the dialog, but is nowhere free()d.  About 2K bytes leaks on each dialog use.

This fix free()s the list using the magic routine `GTextInfoArrayFree()`.