Brush tool : Replace Gtk::Table (deprecated gtk container)

* Gtk::Box as main container
* Gtk::Grid for brushes container
Brush tool : Replace ToggleButton by RadioButton

All the brushs can now be selected by RadioButton holded by a unique group. So only one brush can be selected at the same time.
Brush tool : Fix 605 - Brush do nothing without image layer

If not image found, add one.
Brush Tool : #615 - Brush tool do not (always) used selected color

Fix rgb to hsv conversion