Open a file selection dialog and make it modal.
Return when either the Cancel button is clicked or when a file is
selected.
Default_Dir is the directory to display in dialog initially. Note that
it must end with a directory separator ('/' or '\', depending on your
system). You can use GNAT.Os_Lib.Directory_Separator to get the correct
value for your system.
If Must_Exist is True, then the file (or directory if Dir_Only is True)
must exist.
If Dir_Only is True, then the dialog is modified so that the user can
only choose a directory name, but not a file name.
The value returned is the name of the file selected, or "" if none.