Add code in dlg_filesel_set and dlg_fontsel_set which makes them

Add code in dlg_filesel_set and dlg_fontsel_set which makes them
duplicate the strings they pass to gtk_entry_set_text. I was already
doing that in dlg_editbox_set, but forgot to add the same code when I
revamped FontSpec and Filename to contain dynamically allocated
strings (r9314 and r9316 respectively). This fixes a bug where, on
some versions of GTK (but apparently not up-to-date versions), loading
a saved session causes gibberish to appear in file-selector edit boxes
accompanied by a valgrind error.

[originally from svn r9456]
[r9314 == 9c75fe9a3fa5e6b709f1c210795d1140ca1be2e8]
[r9316 == 62cbc7dc0b33808dc8794c59f60971fbba97894b]