Allows to specify various "regions" of the image with masks, and to correct
for hue, saturation and lightness.
Inspired by the existing L*a*b* grid (in turn taken from darktable)
- 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
