Place newly created modules directly under the cursor

[samuellwn]

This PR calculates a better position for new modules. The old method could be badly wrong, especially in multi-monitor setups. I only tested on Linux.

[samuellwn]

This doesn't account for zoom. If the graph has been zoomed in or out the position will be inaccurate.

[drobilla]

Thanks, I'll check it out. I had noticed that the module position is wonky sometimes but hadn't looked into it (largely because I'm in the process of rewriting the whole canvas anyway, but that will still be a while...)