Blender Git Loki

Fix: "Speaker" Icon for "Muting" in GPencil Dopesheet mode was confusing and used incorrectly

In the other Dopesheet modes, the "Speaker" icon was used to refer to
"animation playback muting", while for GP layers, this was being incorrectly
abused for "layer visibility in viewport".

This commit fixes that by making the following changes:
* A new "eye" icon toggle is added for controlling GP Layer visibility
* The "speaker" icon toggle now controls "Lock Frame to Current" functionality,
which functions more like the "animation playback muting" that is generally
expected.