Blender Git Loki

GPencil UI: Streamline toolbar options a bit

As suggested by mendiobox:

* Don't show "enable editing" in the 3D view. You can already do this by switching
into stroke editing mode here, so no need for the duplication. (In other editors
though, this can't be done yet, so we don't do it)

* Make the "Convert" button into a dropdown so that you don't need to deal with a
a separate popup menu

* In the 3D view, don't show the selection + transform operators that can be easily
found in the menus too (as well as having commonly used shortcuts)