Bugfix in LabGrid class + code refactoring

- suppressed trailing `_` char for class' parameters
- renamed function name (those not related to Gtk) to lowerCamelCase
- changed the mecanism of dragging point, to avoid throwing unecessary
`panelChange` event
- added a `reset` function, with support of `toInitial` to get back the
values at load time (CTRL+click on reset button)
- `on_draw` optimisation: now LabGrid IS A BackBuffer

rtgui/colortoning.h

@@ -12,6 +12,7 @@
 #include "curveeditorgroup.h"
 #include "thresholdadjuster.h"
 #include "colorprovider.h"
+#include "labgrid.h"
 
 class ColorToning final :
     public ToolParamBlock,
@@ -105,7 +106,8 @@ private:
 
     rtengine::ProcEvent EvColorToningLabGridValue;
     Gtk::Button *labgridReset;
-    Gtk::DrawingArea *labgridArea;
+    LabGrid *labgrid;
+    Gtk::HBox *labgridBox;
 
     IdleRegister idle_register;
 };