Color picker UX - allow swicth mode on canvas

[TurboGit]

Allow Ctrl+Clik on canvas to switch between picker point/area.

Ensure that the point position is preserved by using a new variable (pdrag) when dragging a box corner.

This rework also make it possible to switch between point/area for all pickers.

Also use routine to initialize the picker point/area default at a single place. Better than having the same code duplicated in many places.

Rework the picker initialization that do not require anymore a check for some values as the picker point/area are now always initialized at creation time.

Finally a new type dt_pickerpoint_t is introduce to store position of a picker point (we already add dt_pickerbox_t for an area).

Closes #17887

[TurboGit]

@jenshannoschwalm : I finally got it :)