Blender Git Loki

Fix T39895: Displaying the armature layers popup in edit mode always fails.

In fact, armature layers operators (set layers, and show all) were kind of messy and broken
in Edit mode (Select layers had two different operators for Pose and Edit modes, both
using the same funcs that could only work in Pose mode, Show All was supposed to be
used in both modes but again, its exec code could only work in Pose one).

Fixed that by:
* Using only one op for each task, for both modes (with adapted poll func).
* Replacing 'object from context' access by an helper that returns the right Armature
object in both Edit and Pose modes.