Improve reset mesh in setup mode to keep mesh deform keyframes

When you press the reset button in Setup mode it currently does this:
- remove all deformations in setup pose restoring the mesh
- remove all weights
- remove all keys
There is a prompt that asks if you want to remove the weights, and then one that asks if you want to remove the keys. If you choose not to remove the weights or the keys, the mesh doesn't reset.

We could possibly improve this behavior, so that:
- if you want to just reset the mesh, it does exactly that, moving the vertices to their original positions, each vertex would still be keyed in the animations and retain the same weights values.
- if you want to remove the weights but keep the keys, it restores the mesh (you can keeps the mesh deformation if you remove the weights from the weights view), removes the weights, and keep the deformation keys so that each vertex keeps the deformation key which is relative to the new setup pose and the old weights now removed. (this could mean things may get shifted)
- if you want to reset the mesh, remove the weights, and remove the keys, the current behavior already does this.

Reference thread:
http://esotericsoftware.com/forum/Reset-a-mesh-should-not-remove-keys-13715