[x] Selecting multiple anchors by dragging a rectangle
[x] Moving selected anchors via keyboard
[ ] Dragging anchors for images (with default aspect ratio preservation, unless Ctrl?)
[ ] Will need to extend images to have a second element in pts
[ ] Should Shift behave differently, e.g. preserving aspect ratio?
[ ] Should Alt change all rect/ellipses' anchors to preserve center?
[ ] Ability to click on object (not anchor) to restrict to that object's anchors (maybe others grey out), in particular when multiple anchors are overlapping
[ ] Maybe also Shift-click to select multiple objects in this way
[ ] Selecting object(s) in Select tool, and then switching to anchor select, should bring that selection over
[ ] Selecting object(s) in Anchor Select tool, and then switching to Select tool, should bring the selection over
Split off from #35
pts