4-week hiatus and v1.20a en-route

Hello readers. After 6 months of working on PixelCNC just about everyday I needed a little bit of a breather, and am now back on track tackling all the bigger todo list action-items. Here's what's being worked on for the next release:

- leave-stock parameter for pertinent operations, so that roughing operations can leave some meat for a finishing tool to come back around and take care of

- cutting tool diagrams which reflect a visual representation of a tool's dimensions/parameters

- addition of a 3rd cutting tool: a v-bit. technically this can all be achieved via the tapered cutter tool type with a tip diameter of zero, but for some the flute-length method of entering dimensions is counter-intuitive when their cutter is specifically a routing/graving v-bit that only has an angle and diameter

- GPU powered depthmap Minkowski sum to speed up what is currently a CPU operation that is used throughout PixelCNC, especially for simulation preview

- build upon the existing simulation preview setup and implement an actual timeline that can be scrubbed to see a toolpath cutting as a function of time, or some multiple thereof. This is made possible by the CPU image operation that will be moved to GPU.

- UI tooltips so helpful usage information and basic descriptions of the various buttons and parameters can be quickly and easily presented to the user to assist learning and familiarizing themselves with the interface

...and that's barely scratching the surface of the todo list, but seems like the best things to be working on right now. Questions/comments/concerns/suggestions/feedback/criticisms are welcome!