[dylgczqf] Eliminate mouse drag

Eliminate the drag-with-button-down as a mouse UI gesture, because it is awkward to do, and doesn't easily translate to mouse alternative input devices (e.g., touch screens).  It also makes ambiguous whether something is a click (with mouse intended to be still but not quite) or drag (with mouse only moving a very small distance).

All UI interaction should be possible with clicks only.  Drag operations could be available as a shortcut or alternative to another way of doing it without drag.

Inspired by the difficulty of using a VNC client on Android.