Global state idea: Tool-level state

[ctrlcctrlv]

Having the corners, show_sel_box and the new vws_previews and vws_contours in the global state seems hacky, I think I need to come up with some way to have tool-level state, which is set at the time of tool switch.

So maybe each .rs file in events can define a thread-local state variable, containing a different type, and TOOL_DATA can become a reference to the current tool's data, or something.

Maybe tool data state variables should be a HashMap. Gotta think more about this.

[ctrlcctrlv]

Or maybe it's not a problem to call thread_local! in each events/pan.rs, events/pen.rs etc., and each tool just knows which global variable is its own because it defined it. So pull that job out of state.rs and into the different events/ files. Thoughts?