Replaces the following deprecated Gtk classes throughout the codebase by their non-deprecated counterparts. Style, HBox, VBox, HPaned, VPaned, HScrollbar, VScrollbar, VSeparator, HSeparator, Stock, Table, VScale and HScale.
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
