Anchor dragging tool

[edemaine]

Split off from #35

• Dragging a single anchor
• Toggling multiple anchors and dragging them
• Selecting multiple anchors by dragging a rectangle
• 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