Brush tool : Autoresizable Brush chooser as Gtk::ToolPalette

* Remplace Gtk::Box by Gtk::Grid as main container (Gtk::Grid is a new widget that'll replace Gtk::Box)
* Remplace Gtk::Grid by Gtk::ToolPalette for brushes container
* Remplace Gtk::RadioButton by Gtk::RadioToolButton
ToolBox : Propagate Scroll Event from Palette

Make the Tool Palette scrollable from inside the palette not just from the scrollbar
Brush tool : Replace RadioToolButton by ToggleToolButton

Active brush was no-efficient + constitency ui with ToolBox

* Convert all RadioToolButton ref to ToggleToolButton
* Remove unuseful Gtk::RadioToolButton::Group references
* Use of StateBrush_Context::selected_brush_button to track previous selected brush
Brush Tool - Fix Automatic created image can't be embed (related to #722)

* Check for filename param
* created #images directory
* save surface
* set layer's filename param and description
Image Layer : Generate Unique Image Name

* Move generate_new_name from LayerCopy to Instance class
Image Layer : Generate Unique Image Name #2

* Unique Image Name can be based on Layer Description (if filename is empty)
Brush Tool - Fix Automatic created image can't be embed (related to #722) #2

* Now filename are Unique (based on layer description) by Instance::generate_new_name
Fix 730 - Render : Extract Alpha in cairoPng render twice same file.
Fix #728: Magick++ target not working on Ubuntu 14.10

http://www.synfig.org/issues/thebuggenie/synfig/issues/728
Fix #729: FFmpeg target not works on Ubuntu 14.10

http://www.synfig.org/issues/thebuggenie/synfig/issues/729
Fix 555 - Current Time in Canvas Window color not updated when acting on keyframe

* Inform that something has changed about the current time for both
add/remove/change a keyframe by firing canvas_interface::signal_time_changed