Add g_desktop_app_info_get_string_list(); fix g_key_file_free()

The X-Flatpak-RenamedFrom key is used in .desktop files to identify past names for the desktop file. It is defined to be a list of strings. However, there was previously no correct way to retrieve a list of strings from the GKeyFile wrapped by GDesktopAppInfo, short of re-parsing the file with GKeyFile.

Add g_desktop_app_info_get_string_list(), a trivial wrapper around g_key_file_get_string_list(), similar to g_desktop_app_info_get_string() and co.

The GKeyFile patch fixes a bug I happened upon due to the way the gdesktopappinfo test was put together. I suggest that it be cherry-picked to the stable branch(es) if it is merged to master.