Blender Git Loki

GPencil Layer Management: Lock/Unlock All + Isolate Layer

This commit introduces a few operators to make it easier to perform a few common
layer-manipulation operations. Some of these have been sorely needed for quite
a while now...

* Lock/Unlock All - Just as their names suggest, these operators will lock and unlock
all layers in the GP datablock. This is a quick way to unlock all layers previously
locked. These can be found in the new dropdown which replaces the old "Duplicate"
below the +/- (for adding/removing layers); also featured in the dropdown are
the "Duplicate Layers" operator, as well as the show/hide ones.

* Isolate Layer - This operator makes it easy to focus on just a single layer (e.g. the
outlines for a particular character). The "star" button affects editability, while the
"eye" below it toggles editability + visibility.

If any layer is visible/unlocked, this operator will lock and/or hide all; otherwise,
it will unlock/unhide all (to reverse the previous operation).