Drive Joints opens on the picked joint; ui.start takes params (#1297)

Drive Joints opens on the joint picked in the browser, else the first joint that moves, and won't start on a rigid joint. ui.start takes params: joint.drive opens on a joint (id or name) at given values, driving nothing until OK; a command or param it doesn't take is refused.

docs/book/src/guide/assemblies.md

@@ -39,7 +39,8 @@ part still has with its loops closed: the moving parts of a four-bar have one be
 loop locked by a rigid joint has none. The Drive Joints dialog says when a drive stops short and
 why. While it previews, a row under the values gives the value the joint reached and the reason
 (the loop that can't close further, or the two parts a contact set keeps apart), and after OK the
-status line repeats it. The value box keeps what you typed.
+status line repeats it. The value box keeps what you typed. The dialog opens on the joint
+picked in the browser, or on the first joint that moves when none is picked.
 
 The solver places what it reaches from the root and from grounded components. A chain of joints
 that is joined to nothing grounded is never placed, so driving one of its joints moves nothing.