Separate the ability to select from the one to edit in the viewport

**Problem:**
Selecting elements in the viewport is convenient, but it can lead to accidentally editing/moving images/vertices/bones.

**Possible solution:**
Separate the selectability from the editability.
Perhaps by adding a new row to the tools section.

Another solution may be to enable this per element.

This could also be useful for people who were wishing for a skeleton locking feature that want to avoid accidentally ruining the setup pose. In this context, it could be an option to enable or disable in the skeleton properties (just like the export checkbox), so that it can be saved on a per project basis, and it may prevent from editing the setup pose and the weights but allow animation.

**Related forum thread:**
http://esotericsoftware.com/forum/Selecting-bone-image-on-canvas-without-being-able-to-move-it-14153