- 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