Fix Apply Flipped

[jasperges]

It seems that 'Apply Flipped' doesn't work reliably for every rig. Need some more testing.

[sybrenstuvel]

Be sure to update Blender to 14e7ba0c8ac, as there was actually a bug in Blender itself. Hopefully it's a safe enough fix to release in 2.79b, but that still remains to be seen.

[jasperges]

It seems like it was caused by the sloppy-slammed-together-test-rig which didn't have the proper 'roll' set on the bones. The root bone for example had a roll of 270deg. That messed up the flipping.
I will do more testing (with a proper rig this time) and then probably close this issue.

[sybrenstuvel]

We should probably create our own workaround for the fix in 14e7ba0c8ac, as that fix won't be in 2.79b (as it's not a regression bug in 2.79). We'll probably have to check the bone selection ourselves, and if there are bones selected, but without overlap with the bones of the pose library itself, we shouldn't call the apply_pose operator.