Fix 692 - Crash Opening Graph Panel

* remove "get_window()->get_update_area()" call
Graph Panel : Fix curves line witdh
Graph Panel : Fix scrolling
Fix 693 - Graph panel, mouse wheel should just scroll ...

Now,
* MouseWheel --> Scrolling
* Ctrl+MouseWheel --> Zoom
Typo
Fix 708 - Panel contextual menu (close) is empty

item->show was missing.
Improve skeleton deformation
Z-Order field for Bone
Use bone z-order in skeleton deformation layer
Merge branch 'gtk3'

* gtk3: (206 commits)
  Use bone z-order in skeleton deformation layer
  Z-Order field for Bone
  Improve skeleton deformation
  Fix 708 - Panel contextual menu (close) is empty
  Typo
  Fix 693 - Graph panel, mouse wheel should just scroll ...
  Graph Panel : Fix scrolling
  Graph Panel : Fix curves line witdh
  Fix 692 - Crash Opening Graph Panel
  Fix width tool
  Fix keyframe length bug
  Fix group transparency
  Fix behavior of star (and others) linked to spline
  Fix tangents handles
  Track transforamation linked to spline
  Split tangents when link to spline
  Track spline tangents linked to other spline
  Fix tangent editing
  Fix bug with linked vertices
  Link tangents and widths to spline
  ...
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