Rewrote GtkTextMark implementation and adding full methods
Rewrote GtkTextMark implementation and adding full methods

- Previous implementation of GtkTextMark was not really matching Gotk3 structure type (not descendant of GObject)
Gtk3 Specifications say in Object Hierarchy section:
    GObject
    ╰── GtkTextMark
Now, it's ok.
- All methods linked with this object have been wrapped
- there is no change for previously written source code using GtkTextMark object.
- 'gtk.go' and 'gtk.go.h' has been modified to work right with these modifications (hfmrow-patch-7 and hfmrow-patch-8).
gotk3/gotk3,gotk3/gotk3