v2 code quality: Event initialization is messy, needs lots of files to be touched

[jdittrich]

Not a high priority or severity but:

When changing how an element gets added to the canvas, I need to touch:

• scr/app.js (binds jquery drop event to element creation)
• lib/init.js (sets up sidebar and event callback for add-to-canvas )
• src/sidebar/sidebarElements.js (creates sidebar elements, attaches the callback from init.js)

This makes code changes error prone.