Wanted relative control in exposure/color boxes

[stevefal]

Currently mousedown in the exposure or color boxes causes the value to jump to the mouse cursor position immediately. This makes tweaking the current value difficult because the user must first align the mouse cursor precisely to the current value, or lose the current value on mousedown.

Suggestion is to support relative control such that mousedown in a box does not set the value directly, but establishes a zero-delta point from which any movement causes a corresponding change in the current value. For example, mousedown anywhere in the exposure box does nothing, but moving up/down from that point moves the current value up/down.

If absolute changes are desired, that could be supported via double-click, such that a double-click causes the current value to jump to the mouse cursor position, from which any further movements cause adjustments.