Improve Path tool layer and point selection behavior

[Keavon]

• Click-dragging to box select from inside a shape (even if it's not filled) should not select all the shape's points and begin dragging them; it should allow the box selection just like outside the shape (Add segment editing mode to the Path tool #2712)
• Our current way of clicking another layer to make it selected should not happen unless it's a double click (exception: if no layers are selected, single-click should work to select the first one). Without Shift, it transfers the selection to the not-yet-selected double click target object. If Shift is held, it adds or removes that layer to the selected objects. When added, its points will always start initially deselected.
• Double-clicking an already selected layer, without Shift held, will select all its points. Other selected layers are unaffected (they keep their point selections and layer selection doesn't transfer away).
• Alt + double-clicking an already selected layer will deselect all its points.
• TODO: more changes?