This PR adds support for text selection by dragging with a mouse. The current implementation only supports horizontal dragging since vertical dragging will most likely conflict with scrolling.

The solution is to make it so that scrolling is only triggered when initiated by touch events, and vertical text selection is only triggered by a mouse. No conflicts!

But with the current state of Flutter, we can't disable mouse-drag-to-scroll yet because there is no other way to scroll on desktop. Once wheel scrolling is fully implemented, we can revisit this.

• wheel scroll.
• scroll bars.
• disable drag-to-scroll when the device is a mouse.
• enable text selection by dragging in any direction.