Use a const reference instead of a const pointer.

The argument is mandatory, so use a reference.
Use a reference instead of a pointer.

The argument is mandatory, so use a reference.

This changes the callbacks of the GUI, thus causing several changes in
all users of the callbacks.
Use a reference instead of a pointer.

The argument is mandatory, so use a reference.

This updates get_parent.
Rename gui2::twidget::dirty to is_dirty.

Also updates the setter and getter members of gui2::twidget.
Rename gui2::tcanvas::dirty to is_dirty.

Also updates the setter and getter members of gui2::canvas.
Remove the default argument of set_is_dirty.
Merge branch 'gui_polishing'