Selecting and moving multiple points works properly only for points which belong
to a single object. Multiple points from >1 object can be selected, and some
operations can be applied successfully (deleting, convert to curve), but moving
the points only affects the object whose point is being moved.

Steps to reproduce:
1. Create two lines
2. Select both lines
3. Edit > Points. All four endpoint handles should be visible.
4. Select one point from each line (click one; shift-click the second)
5. Move either selected point: only that point will move, even though both
points are still selected.

Expected result:
All selected points should move together--this is what happens for other
operations in this situation (e.g. deleting)