Allow a toolkit to provide its own version of UI dialogs. * plot/modules.mk (plot_PRIVATE_FCN_FILES): Add __file_filter__.m, __is_function__.m, __uigetdir_fltk__.m, __uigetfile_fltk__.m, __uiputfile_fltk__.m. * plot/uigetdir.m: Rework to remove FLTK-specific stuffs and allow use of toolkit-provided dialogs. Fallback to FLTK dialogs. * plot/uigetfile.m: Likewise. * plot/uiputfile.m: Likewise. * plot/private/__fltk_file_filter__.m: Assumes input is now always a cell array of strings. * plot/private/__file_filter__.m: New file. * plot/private/__is_function__.m: Likewise. * plot/private/uigetdir_fltk__.m: Likewise. * plot/private/uigetfile_fltk__.m: Likewise. * plot/private/uiputfile_fltk__.m: Likewise.