Fix: Remove most of the code for treating GP as a special case in the vertex group poll functions
These were causing crashes on GP datablocks with ob->data not set when clicking on the Vertex Groups panel (i.e. as the operator lookups would cause problems)
GP Branch: Use ob->data instead of ob->gpd for storing GP object's GPencil datablock
WARNING: Old files created in the GP branch may break after this commit!
* There won't be data loss, but it will be necessary to manually fix this per-file as needed. The old ob->gpd pointer remains for now to ease the transition.
* The code will attempt to version patch old files that store data in this pointer. However, right now, this only works for GP Objects and for Empties.
Hacky WIP code to verify that GPencil-Dopesheet Integration (for modifiers) is working
This code should be replaced, as it currently just duplicates the modifier-looping logic (bad!). However, this is just a temporary placeholder needed to get the updateDepsgraph() calls to run at all.
In any case, this would've been easier if I'd done the ob->gpd ==> ob->data fixes first, since this hack wouldn't be needed.