This patch is for get better modal window support in GTK+. It allow an
easier method to get modal window and correct the problem with standard
dialogs (file_selection and color_selection) that doesn't work as
when they are grabed.

This patch adds some changes in these files:

gtkwindow.[hc] -> Adds modal field and gtk_window_set_modal function
	to allow set a window to work as a modal one. gtk_window_show
        check if modal is active and if it is true grab the window.
        gtk_window_hide does the inverse action.

gtkfilesel.c -> Test if it is running in modal form and in that case
will run "Create Dir","Delete File" and "Rename File" dialogs as modal
too. In actual gtk+ if file selection is running modal widgets in
"Create Dir","Delete File" and "Rename File" dialogs doesn't work.

testgtk.c -> Added a button with an example of creating modal dialog
boxes. Including file selection and color selection standard dialogs.
This is FAQ in gtk-list so I think this example will be useful.