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. (Improve Path tool layer selection behavior using double-click instead of single-click #2794)
• 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). (Improve Path tool layer selection behavior using double-click instead of single-click #2794)
• Alt + double-clicking an already selected layer will deselect all its points. (Improve Path tool layer selection behavior using double-click instead of single-click #2794)